Consolidate CI on GitHub Actions

[mvdan]

I see you use a mix of different CI platforms to test on Windows and Linux. We can consistently test on Linux, Mac, and Windows for free on GitHub itself.

There is no limit for build minutes for OSS; there's just a limit of 20 concurrent jobs. Even when testing on two Go versions on three OSs, we'd basically never hit the limit.

If you want to see an example of them in action, I have a sample setup with some docs at https://github.com/mvdan/github-actions-golang.

If this sounds like a good idea, I can work on it.

[mpvl]

It would indeed be nice to consolidate testing, or to have OS X testing at all!

[cueckoo]

This issue has been migrated to cue-lang/cue#144.

For more details about CUE's migration to a new home, please see cue-lang/cue#1078.