Test nix builds as well

[turion]

There is an all-ghcs.nix file which tests whether the package can be built and tested in nix for several versions of GHC. This should be tested on CI as well, ideally coherently with the tested-with section in clay.cabal.

Detailed to do

• Move Nix setup to flakes
• Update all-ghcs to the current versions tested on CI
• Build the all-ghcs derivation on CI. Probably, https://github.com/cachix/install-nix-action