build: use the system provided LuaJIT if found #8133

Job	Run time
run-ubuntu-unit-tests (-DFLB_JEMALLOC=On, gcc)	20m 16s
run-ubuntu-unit-tests (-DFLB_JEMALLOC=On, clang)	18m 46s
run-ubuntu-unit-tests (-DFLB_JEMALLOC=Off, gcc)	18m 15s
run-ubuntu-unit-tests (-DFLB_JEMALLOC=Off, clang)	17m 38s
run-ubuntu-unit-tests (-DFLB_SMALL=On, gcc)	18m 59s
run-ubuntu-unit-tests (-DFLB_SMALL=On, clang)	20m 2s
run-ubuntu-unit-tests (-DSANITIZE_ADDRESS=On, gcc)	18m 43s
run-ubuntu-unit-tests (-DSANITIZE_ADDRESS=On, clang)	18m 1s
run-ubuntu-unit-tests (-DSANITIZE_UNDEFINED=On, gcc)	18m 46s
run-ubuntu-unit-tests (-DSANITIZE_UNDEFINED=On, clang)	18m 31s
run-ubuntu-unit-tests (-DFLB_COVERAGE=On, gcc)	54m 33s
run-ubuntu-unit-tests (-DFLB_SANITIZE_MEMORY=On, gcc)	18m 15s
run-ubuntu-unit-tests (-DFLB_SANITIZE_MEMORY=On, clang)	18m 29s
run-ubuntu-unit-tests (-DFLB_SANITIZE_THREAD=On, gcc)	18m 16s
run-ubuntu-unit-tests (-DFLB_SANITIZE_THREAD=On, clang)	17m 47s
run-macos-unit-tests (-DFLB_JEMALLOC=Off)	14m 14s
run-macos-unit-tests (-DFLB_SANITIZE_MEMORY=On)	9m 58s
run-macos-unit-tests (-DFLB_SANITIZE_THREAD=On)	10m 37s
Unit tests (matrix)	1s
	5h 50m 7s

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