Add support for Github actions-based CI

[toots]

This PR adds support for CI using github actions, following a discussion in #20271 (comment)

Example of a full run: https://github.com/toots/opam-repository/actions/runs/1620485627

Implementation details

• The builds are made from scratch, using official docker images for distributions or github runners and the standard ocaml/setup-ocaml@v2 and etc. No need to maintain a matrix of docker images
• OCaml builds are cached and take about 1m40s
• Most features from the current CI are supported except:
  • The opensus image was buggy and had to be dropped
  • lowerbound tests aren't implemented as it seems that the required solver is not (yet) available with the opam build shipped by ocaml/setup-ocaml@v2
  • Architectures other than the one provided by the runners (linux/mac, amd64) are not supported. Windows could be but the dance between official opam packages and windows packages does seem to make this painful.
  • extra are supported based on available ocaml-option-* packages.

Overall, this seems to be an interesting option as this is free and scales well. It could be nice to have it along the official CI to see how it behaves and, perhaps, use it to alleviate some of the pressure on the current CI.

[kit-ty-kate]

As explained in the "discussion" you linked above, we are not looking to use github actions.
I feel a bit bad to close this but I hope you didn’t wasted too much time on this PR on this christmas day.

Github actions only supports up to 20 concurrent jobs, our cluster supports up to a 1000, a simple revdeps run on dune or other heavily used package would simply kill github actions and for simple packages this would result in the same behaviour as opam-repo-ci.

As said previously, if you have ideas on how to improve opam-repo-ci you can open feature requests at https://github.com/ocurrent/opam-repo-ci/issues.
If you want to have a voice chat with us feel free to hit me up on Discord or other messaging system of your choice.

[toots]

Haha. No, no problem. It was a good exercise for me to understand both github actions and how the CI works here.

I still think this could be added at no cost and provide and alternative to the main CI, I don't see what would be lost by adding it.

Happy holidays!

[kit-ty-kate]

I still think this could be added at no cost and provide and alternative to the main CI, I don't see what would be lost by adding it.

It still brings quite a heavy maintenance and duplication cost as well as confusion for users and a difficult to navigate interface for us maintainers (having to scroll a list of 1000s of PR checks that we would not use just to get the results from opam-repo-ci would be quite painful).
I would much rather focus our efforts into improving what we have. For instance here is a proposal to improve the UI and split core checks and revdeps checks to improve readability for users: ocurrent/opam-repo-ci#127