[ci/msys2] Add a job building in the clang/llvm environment Clang is more strict in certain areas and will fail, like https://github.com/harfbuzz/harfbuzz/pull/5079#issuecomment-2673805310 https://github.com/harfbuzz/harfbuzz/issues/5197 * Add CLANG64 to the matrix * Install the virtual "cc" package which pulls in the default compiler for the respective environment * Remove gcc-libs, it should be pulled in automatically * Depend on MSYSTEM_PREFIX for deleting files to be environment agnostic