Update GitHub actions workflow to build for macOS

[hach-que]

The GitHub actions workflow should build LLVM/Clang for macOS as well: https://github.com/RedpointGames/llvm-project/blob/redpoint/18.x/.github/workflows/redpoint.yml

We probably need instructions on how to get UBT to use an alternative Clang on macOS as well. As far as I can tell, the only way to make it use alternative Clang on macOS would be to copy the Clang binaries over the top of the ones that ship in the Xcode toolchain folder.