CI: Use llvm-tools-preview toolchain instead of external llvm-tools toolchain

[briansmith]

• In mk/{install-build-tools.sh, should not do install_packages llvm-$llvm_version. Instead we should install the tools using rustup +$toolchain toolchain component add llvm-tools-preview before the coverage runs. This would require us to parse "+$toolchain" out of the command line, much like we parse "--target=$target". We'd need to update ci.yml accordingly to pass +${{ matrix.rust_channel }} on the command line. In ml/cargo.sh, instead of using external LLVM tools for code code coverage analysis, we should use the llvm-tools-preview tools installed in the previous step.
• In mk/check-symbol-prefixes.sh, we should similarly switch to using the llvm-tools-preview llvm-nm, and change ci.yml accordingly.
• We should change the conditional logic in ci.yml so that mk/check-symbol-prefixes.sh is run for apple targets, where it is currently being skipped.
• We should change the conditional logic in ci.yml so that mk/check-symbol-prefixes.sh is run for Windows targets. (We can force the use of bash as the shell for Windows). This may require tweaking some of the logic.
• We should change the conditional logic in ci.yml so that mk/check-symbol-prefixes.sh is run for all remaining targets, if at all possible.

Each checklist item above should be done as a separate PR, ideally.

[briansmith]

PR #1908 attempts to resolve the mk/check-symbol-prefixes.sh parts.

[briansmith]

• mk/{install-build-tools.sh, should not do install_packages llvm-$llvm_version

It still needs to, but only when we are in $use_clang mode.

Instead we should install the tools using rustup +$toolchain toolchain component add llvm-tools-preview before the coverage runs. This would require us to parse "+$toolchain" out of the command line, much like we parse "--target=$target". We'd need to update ci.yml accordingly to pass +${{ matrix.rust_channel }} on the command line.

Done in PR #1980.

In mk/check-symbol-prefixes.sh, we should similarly switch to using the llvm-tools-preview llvm-nm, and change ci.yml accordingly.
We should change the conditional logic in ci.yml so that mk/check-symbol-prefixes.sh is run for apple targets, where it is currently being skipped.

Done in PR #1908.