[CI] Add snap package

[gares]

Request for aliases (coqide and coq_makefile) is ongoing here: https://forum.snapcraft.io/t/aliases-request-for-coq-prover/21925

@MSoegtropIMC this is now ready for review. There are a few things to document:

• the package is not just bundling files already present, it runs the platform script from the snap building infrastructure (unlike we do on windows, hence the job is not on top of the ubuntu one), hence...
• the job builds a minimal platform unless told otherwise, and does not upload the snap in the store. One has to trigger a manual build. On the default branch of the repo this can be done using a web UI, but for other branches one has to do a call to the API of github. The script github_actions/run_snap.sh can do it. The idea is that the job is run "in full" only when one wants to upload to the store, otherwise a minimal (quicker) thing is build but not uploaded. Saving the artifact is orthogonal to uploading to the store. Uploading to the store does not make the snap public, there is a pipeline (I still have to test) to move from edge to beta and then to stable a package by hand. So it is not the end of the world if one uploads by mistake, but it is just a waste I believe.

A few point to discuss:

• the layout: I have linux/create_linux_installer.sh but also snap/ containing templates and the result of calling the first script, is it ok? In general this PR creates 3 directories, maybe it's too much.

Improvements we could do:

• some code, like the one generating the contents of the snap (the description) and the one cleaning up unnecessary files, could be shared.

The package is still private, here a preview:

[MSoegtropIMC]

Except for the editorial comments, this looks good to me. Thanks!

[gares]

This is finally ready. I'll merge it and ask for testing. The package in online, although not listed in the search engine.

[Zimmi48]

When github/roadmap#167 is implemented (excepted before the end of March), it should help to deploy from a CI run.