mac-mono: fix x64arm64/x64ARM64 case issue

[jsm174]

Changes

This PR fixes not being able to build macos x64 + arm64 builds with ubuntu.

It fixes https://github.com/game-ci/unity-builder/issues/320

This is a bug in Unity, and if/when fixed, this patch should be able to be removed:

https://forum.unity.com/threads/linux-command-line-build-of-macos-has-wrong-mono-path.1200499/

Note, I was able to verify the fix implemented in this PR with a custom docker image:

FROM unityci/editor:2021.2.13f1-mac-mono-0
RUN ln -s /opt/unity/Editor/Data/PlaybackEngines/MacStandaloneSupport/Variations/macos_x64arm64_player_nondevelopment_mono /opt/unity/Editor/Data/PlaybackEngines/MacStandaloneSupport/Variations/macos_x64ARM64_player_nondevelopment_mono

I am not sure how to test this PR using the artifacts from the CI.

Checklist

• [x] Read the contribution guide and accept the code of conduct
• [x] Readme (updated or not needed)

[github-actions[bot]]

[webbertakken]

Merging this. It will be available once we roll out the next release.