macOS should be added to arcade-validation builds

While both are Unix-based, macOS is BSD based while GNU-Linux is..............linux-based. This means there can be slight differences in how certain commands (e.g. `sed`) are implemented and makes it important that we test on both.

Furthermore, macOS 10.15 is switching to zsh as its default interpreter, so we'll likely need to test both 10.15 and a version that uses bash.

Adding new phases for these should not expand the build time significantly.

cc/ @riarenas