Consider adding GitHub actions build jobs

[ghost]

As far as I can tell from reading the documentation, GitHub actions should be free for public repositories. GitHub Actions has the ability to build on macOS, which may allow us to distribute newer builds on macOS. We could also run a build job with different options through that.

[timfel]

Yes, I would like to move away from Travis.

Note that we used to have mac jobs on Travis, and I could still enable them again - the issue is not that I cannot run the CI. The issue is that no one who contributed regularly has a mac can fix things when they break, so at some point I just disabled them.