GHA MacOS clang21 job fails due to openMP not found

[KrisThielemans]

For whatever reason, in #1689 the clang21 macos job fails as CMake cannot find OpenMP. Possible solution: https://gist.github.com/scivision/16c2ca1dc250f54d34f1a1a35596f4a0

[gschramm]

This is what I used to do
https://github.com/gschramm/parallelproj/blob/7f599947ea4af29d9081895ff48663a506ea6d02/.github/workflows/c_python_build.yml#L40

    - name: Install openmp on macOS
      if: runner.os == 'macOS'
      run: |
        brew update
        brew install llvm libomp  # Install OpenMP
        echo "CC=/opt/homebrew/opt/llvm/bin/clang" >> $GITHUB_ENV
        echo "CXX=/opt/homebrew/opt/llvm/bin/clang++" >> $GITHUB_ENV

but not sure it that is still working