Fully build release packages in CI

[EwoutH]

Currently (as far as I understand) the release packages are build by hand, following this guide. I would like to propose to implement this process fully automated in CI.

Advantages of packaging fully in CI:

• Builds are fully reproducible
• Release load for maintainers is significantly reduced
• Testing new features, bugs, and dependency updates is far easier, especially on multiple platforms
• We could publish nightly/weekly builds
• It reduces the bus-factor

I personally have experience with GitHub Actions and AppVeyor, so I could help with those two. I tried following the guide once, but got hopelessly stuck. So maybe a joint coding sessions could be a solution for that.