ci: move linux and macos to Github Actions

[svmhdvn]

This change replaces Cirrus CI tasks for linux and macos with Github Actions equivalents.

Github actions doesn't seem to have a "compute credits" or similar restriction for public instances. Since kakoune is mainly hosted on github, it's also really well integrated with the pull request flow.

See https://github.com/svmhdvn/kakoune/actions/runs/7578698028 for a successful run.

[krobelus]

Perhaps the problem with Cirrus was that a hung test (which has been fixed since) used up all available time.
This change makes it so we no longer specify the compiler version explicitly, and moves from g++-10 to g++-11. Both seem fixable if desired.

[svmhdvn]

Perhaps the problem with Cirrus was that a hung test (which has been fixed since) used up all available time.

Sure, but I still find it more valuable to be free from these limits in the first place, and to reduce further dependencies on other services outside of the (implied constant) Github.

Both seem fixable if desired.

Yes exactly, I can easily change the toolchain versions (or add more to the matrix) if desired by kakoune's maintainers.

[krobelus]

Looks like Cirrus works again.
In the meantime we also added Sourcehut CI which covers everything except macOS.
The status quo seems acceptable but let's see what the future brings