MINIFICPP-2744 Pin libc++ version in clang CI job

[lordgamez]

The github actions ubuntu image uses a different llvm repository which overrides the libc++ version needed for the clang-20 compilation. Pin the libc++ version on the runner to avoid the version conflict.

https://issues.apache.org/jira/browse/MINIFICPP-2744

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[szaszm]

maybe we should remove whatever llvm repository github added, or use that instead, if the right versions are present in it

[lordgamez]

maybe we should remove whatever llvm repository github added, or use that instead, if the right versions are present in it

Unfortunately we cannot use it because it contains libc++23 and we need libc++20 to be compatible with the current clang-20 compiler. Removing it would be more tricky as I cannot see the internals of the docker image.