Merge bitcoin#26470: guix: Clean up libexec/build.sh

d3e5779 guix: Drop non-existent directories (Hennadii Stepanov)
43c4afc guix: Drop no longer used `prepend_to_search_env_var()` (Hennadii Stepanov)

Pull request description:

  1. The `prepend_to_search_env_var()` function was introduced in bitcoin@c1ae726, and it has no longer been used since bitcoin@1dd8cbf.

  2. The `${NATIVE_GCC}/lib64` and `${NATIVE_GCC_STATIC}/lib64` do not exist at all.

  Guix builds:
  ```
  0a1ed84ba127853fdeb14e6104e97d853116bf3ac9fa4cdd4b72c428ca738f7a  guix-build-d3e5779f110a/output/aarch64-linux-gnu/SHA256SUMS.part
  c69fddab6384ece4061606109acce2f4f991129b5ad2005e9977bc74ad88f60c  guix-build-d3e5779f110a/output/aarch64-linux-gnu/bitcoin-d3e5779f110a-aarch64-linux-gnu-debug.tar.gz
  b8a0968e7057ed6b1703cc5b5213178b06c51c7649c2639d32523f049b082a69  guix-build-d3e5779f110a/output/aarch64-linux-gnu/bitcoin-d3e5779f110a-aarch64-linux-gnu.tar.gz
  9624c5f331d6e2e4f6bb63a3573ff8f688e2662bf8a1959068511ee76fea1b31  guix-build-d3e5779f110a/output/arm-linux-gnueabihf/SHA256SUMS.part
  4b2fecfac94aa55489a9e7316944a1c0216750bfc8658e8dbe2266ad6abed7cd  guix-build-d3e5779f110a/output/arm-linux-gnueabihf/bitcoin-d3e5779f110a-arm-linux-gnueabihf-debug.tar.gz
  db413d3ed5e4917a236dea10d121953c01812d7f3202bf503fb3a7bb40e9655e  guix-build-d3e5779f110a/output/arm-linux-gnueabihf/bitcoin-d3e5779f110a-arm-linux-gnueabihf.tar.gz
  f4952abe163f536c085a3ad9249840f940f5a2670b77042e2c355886301174a0  guix-build-d3e5779f110a/output/arm64-apple-darwin/SHA256SUMS.part
  3f9b76ed00e90547e980062114de435c10b0253a3fa3bca867758c413f68d465  guix-build-d3e5779f110a/output/arm64-apple-darwin/bitcoin-d3e5779f110a-arm64-apple-darwin-unsigned.dmg
  ec30943bf796c7c55f9d5fb186054070483b888c87d1c79a02ba2828c85d9643  guix-build-d3e5779f110a/output/arm64-apple-darwin/bitcoin-d3e5779f110a-arm64-apple-darwin-unsigned.tar.gz
  31fccaf6ff6f70ef3fe09c592f2630f5b2bc39f5c8eadf4f7660e379389442be  guix-build-d3e5779f110a/output/arm64-apple-darwin/bitcoin-d3e5779f110a-arm64-apple-darwin.tar.gz
  6df55d847ad0f3c4cdd35c3fc9603ab58e44bf76a4a942c4f2f268fa02e710e1  guix-build-d3e5779f110a/output/dist-archive/bitcoin-d3e5779f110a.tar.gz
  1e264b6cd129e1353c271049fd3c8338a1d2183aea847cb4ea9d4095fba56a54  guix-build-d3e5779f110a/output/powerpc64-linux-gnu/SHA256SUMS.part
  03d325d8bffa72e79638ae0e5cd7d6c761cdc5e9d664158c83a5f9ff9f393634  guix-build-d3e5779f110a/output/powerpc64-linux-gnu/bitcoin-d3e5779f110a-powerpc64-linux-gnu-debug.tar.gz
  8d459c2fca6e6712cde54ce030cddae9c3fa0607dd9fb4a951f8d62f3e456162  guix-build-d3e5779f110a/output/powerpc64-linux-gnu/bitcoin-d3e5779f110a-powerpc64-linux-gnu.tar.gz
  0ef58af43e18dc7490ea8f01d392e6645b7f978817a8fb0727d90559680a0105  guix-build-d3e5779f110a/output/powerpc64le-linux-gnu/SHA256SUMS.part
  fe4be346506e5f9cc3c5f8a952f4d4b2ac13a2562854d26deee536f411847189  guix-build-d3e5779f110a/output/powerpc64le-linux-gnu/bitcoin-d3e5779f110a-powerpc64le-linux-gnu-debug.tar.gz
  52c0961bfca3985899f872145c566b5220245096a3edc94347800e1de199a17e  guix-build-d3e5779f110a/output/powerpc64le-linux-gnu/bitcoin-d3e5779f110a-powerpc64le-linux-gnu.tar.gz
  ade3eb3f496a5904d12cb09095e7f05c69961d759aba1acc82e01bcfcc2c6ef1  guix-build-d3e5779f110a/output/riscv64-linux-gnu/SHA256SUMS.part
  05a9d0921b5706e210fb0b5a1a70890604ee22a7bbdc25f011cd1bbff1420120  guix-build-d3e5779f110a/output/riscv64-linux-gnu/bitcoin-d3e5779f110a-riscv64-linux-gnu-debug.tar.gz
  fb897b1b527450da6a033802c218722d69176c2572c321ad3e18b2ef3bf398ed  guix-build-d3e5779f110a/output/riscv64-linux-gnu/bitcoin-d3e5779f110a-riscv64-linux-gnu.tar.gz
  a76da4a5d3fd3ed98adbe1e4f1b1c974937e401801d65b55a14e21339b072f9d  guix-build-d3e5779f110a/output/x86_64-apple-darwin/SHA256SUMS.part
  3eb30a851466d4dcf08fbf1d94071d8d6a253fee52540ab109ae9cf629cb36d2  guix-build-d3e5779f110a/output/x86_64-apple-darwin/bitcoin-d3e5779f110a-x86_64-apple-darwin-unsigned.dmg
  6a350a4ea7afbdfba07d1d51039d9c3da5f5ff0a3d4e40095006d0cb571a5c65  guix-build-d3e5779f110a/output/x86_64-apple-darwin/bitcoin-d3e5779f110a-x86_64-apple-darwin-unsigned.tar.gz
  32761153c68431c3bf1eb6638caa00bd4d7daf7372cdea2a9cc711c4c1f68326  guix-build-d3e5779f110a/output/x86_64-apple-darwin/bitcoin-d3e5779f110a-x86_64-apple-darwin.tar.gz
  79fa0b6b25a74c4681679a04ceb9fab1ae0e339ed9e44a30e3c84ee019255e85  guix-build-d3e5779f110a/output/x86_64-linux-gnu/SHA256SUMS.part
  dd496a120f99a818fe453f23f607dd064c855e6d5d199870ce3ab924aca6fed0  guix-build-d3e5779f110a/output/x86_64-linux-gnu/bitcoin-d3e5779f110a-x86_64-linux-gnu-debug.tar.gz
  6dc03d9ad75126ad67c29afbf3ef12e99a97f0792cd8fac6407a2f4ca911f9d1  guix-build-d3e5779f110a/output/x86_64-linux-gnu/bitcoin-d3e5779f110a-x86_64-linux-gnu.tar.gz
  32fb6e76ed757d9d4693c3ef2cea7bd7b9151cadbf9976ed64ab1e3a7ee71e9e  guix-build-d3e5779f110a/output/x86_64-w64-mingw32/SHA256SUMS.part
  93bbc60e546571e14268e25efcf83375c5ec4ed13c546cbf5b76d849a0f50f35  guix-build-d3e5779f110a/output/x86_64-w64-mingw32/bitcoin-d3e5779f110a-win64-debug.zip
  b85932e7cde8616c2d7342735ad8c6247d556fa86ccee8d0ecd326debeb93d8c  guix-build-d3e5779f110a/output/x86_64-w64-mingw32/bitcoin-d3e5779f110a-win64-setup-unsigned.exe
  f77431e1d5b7a356c065dd81383222116cf8254bda0666a169b25415cfba1810  guix-build-d3e5779f110a/output/x86_64-w64-mingw32/bitcoin-d3e5779f110a-win64-unsigned.tar.gz
  8d1a43386648ef5d97aae806f0e38f7fa3ec5f39b30b495fdcf6be6590a392eb  guix-build-d3e5779f110a/output/x86_64-w64-mingw32/bitcoin-d3e5779f110a-win64.zip
  ```

ACKs for top commit:
  fanquake:
    ACK d3e5779

Tree-SHA512: bd32442c81146a5558a4f5bc79e1eb841e7c920551289a2c910f8a697c0a70ea26926e9ca7cdb486176b772ee42544559939417a2bdf3ab6cc4b65dba816a087

contrib/guix/libexec/build.sh

@@ -69,16 +69,12 @@ unset CPLUS_INCLUDE_PATH
 unset OBJC_INCLUDE_PATH
 unset OBJCPLUS_INCLUDE_PATH
 
-export LIBRARY_PATH="${NATIVE_GCC}/lib:${NATIVE_GCC}/lib64:${NATIVE_GCC_STATIC}/lib:${NATIVE_GCC_STATIC}/lib64"
+export LIBRARY_PATH="${NATIVE_GCC}/lib:${NATIVE_GCC_STATIC}/lib"
 export C_INCLUDE_PATH="${NATIVE_GCC}/include"
 export CPLUS_INCLUDE_PATH="${NATIVE_GCC}/include/c++:${NATIVE_GCC}/include"
 export OBJC_INCLUDE_PATH="${NATIVE_GCC}/include"
 export OBJCPLUS_INCLUDE_PATH="${NATIVE_GCC}/include/c++:${NATIVE_GCC}/include"
 
-prepend_to_search_env_var() {
-    export "${1}=${2}${!1:+:}${!1}"
-}
-
 # Set environment variables to point the CROSS toolchain to the right
 # includes/libs for $HOST
 case "$HOST" in