Test against latest stable and unstable regularly

[iFreilicht]

#850 was caused by us not testing whether PRs (or even master) is compatible with the latest stable release of nixpkgs.

AFAIU, we're basically already testing whether disko is compatible with current latest unstable via the github action (which actually is failing right now 😬), but maybe there's a better way?

[Mic92]

I think having an automatic flake update is already the best way. You don't want to break people's pull request because for some unrelated regressions caused in a different place.
It's up to us to fix flake update pull request.

[iFreilicht]

Okay that works for unstable, but what about the latest stable release? This is currently not covered in any way, is it?