Generally, enable new nixos releases earlier

[dpc]

I think it's been a second time, when we've upgraded to the new nixos branch early, and had to endure unnecessary failed github status checks (not required to pass the CI, but still):

Is there any technical reason these can't be enabled ahead of time? If our project made a decision to try out upcoming NixOS release early (e.g. to make sure everything still works before NixOS release is stabilized, so we have time to report issues), we would prefer for this check to just keep working.

[Jayman2000]

I’m kind of in the opposite situation. I’m not really interested in having Nix Flake Checker allow beta branches at the moment, but I am interested in having Nix Flake Checker disallow deprecated branches. I just opened #151 which will hopefully enable both of our use cases.