a different CXX compiler might be randomly chosen on `macos-*`

Describe the bug
On the same unmodified workflow a different CXX compiler is chosen.

This appears to happen extremely rarely. It is the first time I have ever encountered this.

To Reproduce
Steps to reproduce the behavior:

1. Add a step with a CMake build

Expected behavior
The same compiler is chosen all of the time.

Runner Version and Platform

2.328.0 - macos-15

What's not working?

A different CXX compiler is chosen.

Job Log Output

https://github.com/danmar/simplecpp/actions/runs/17610432219/job/50030955686

-- The CXX compiler identification is AppleClang 17.0.0.17000013
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /Applications/Xcode_16.4.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++ - skipped

https://github.com/danmar/simplecpp/actions/runs/17609753077/job/50028719042

-- The CXX compiler identification is AppleClang 17.0.0.17000013
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/clang++ - skipped

Using the latter causes the following unexpected compiler warning:

error: include location '/usr/local/include' is unsafe for cross-compilation [-Werror,-Wpoison-system-directories]

Start with the two linked job logs and compare compiler resolution during the CMake step on macos-15, using the reported runner version 2.328.0 as context. Determine why the same workflow selects /usr/bin/clang++ in one run and the Xcode toolchain clang++ in another; done means the compiler selection is consistent and the reported cross-compilation warning is no longer triggered.