Upload CI builds as artifacts to Github actions

[wormyrocks]

I see that there's a CI task to make macOS builds. Since there are no official macOS builds anywhere, it would be nice if the output of that task were uploaded as an artifact to the github action: https://github.com/actions/upload-artifact

In particular: I had no problem building mayo on my M1 Mac, but I wanted to send a copy to a friend with an Intel Mac, and ran into issues cross-compiling. Would be much easier to just grab from CI.