Make CI run on Mac as well

[rheitjoh]

Since we have been having some Mac-specific problems, it seems sensible to run the CI on MacOS as well. Github Actions seems to support running on MacOS via the runs-on field. So should not be too hard to add.

[rheitjoh]

Can be easily added via

runs-on: ${{ matrix.os }}

strategy:
    matrix:
        os: [ubuntu-latest, macos-latest]

However, we still use the old (v1.26) Mcl version where the Java FFI makefile's include path is incorrect for Mac OS.
So cannot add to CI at the moment.