Add sponsor button to key repositories

[conejoninja]

trafficstars

Add the sponsor button linking to open collective in the repository.

Needs to be enable ( https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/displaying-a-sponsor-button-in-your-repository )

[deadprogram]

The fix for nix is in... or at least for the CI. Please rebase against latest dev.

[conejoninja]

done, but now Linux/assert-test-linux fails :/

[aykevl]

This PR doesn't change any code, so I'd suggest to just ignore the CI errors. The CI errors are weird though, if they keep happening I think we should do a new llvm-build (by incrementing the cache key) to see whether that helps. It looks like a version mismatch indeed.

[deadprogram]

Thank you @conejoninja now merging.