mamba installation fails on macOS GH runner

[orbeckst]

See, for example, https://github.com/Becksteinlab/GromacsWrapper/actions/runs/6191618372/job/16810197850#step:3:802

Warning: Error while loading conda entry point: conda-libmamba-solver (dlopen(/usr/local/miniconda/lib/python3.11/site-packages/libmambapy/bindings.cpython-311-darwin.so, 0x0002): Library not loaded: '@rpath/libarchive.13.dylib'
    Referenced from: '/usr/local/miniconda/lib/libmamba.2.0.0.dylib'
    Reason: tried: '/usr/local/miniconda/lib/libarchive.13.dylib' (no such file), '/usr/local/miniconda/lib/python3.11/site-packages/libmambapy/../../../libarchive.13.dylib' (no such file), '/usr/local/miniconda/lib/python3.11/site-packages/libmambapy/../../../libarchive.13.dylib' (no such file), '/usr/local/miniconda/bin/../lib/libarchive.13.dylib' (no such file), '/usr/local/miniconda/bin/../lib/libarchive.13.dylib' (no such file), '/usr/local/lib/libarchive.13.dylib' (no such file), '/usr/lib/libarchive.13.dylib' (no such file))

[orbeckst]

This may be an upstream issue.

I made macOS a non-mandatory status check for right now.