prepare for garnix

[dermetfan]

Description

• Add garnix cache to the flake nix config.
• Add garnix.yaml that builds only .#checks.
• Adjust the flake outputs to make garnix build everything that Hydra did.

Garnix does not build the hydraJobs output
so move them all into the checks output,
adhering to the flake output schema
because garnix does not build nested attrsets.

Note that this replaces the layout of the checks output
but it should still include all the checks that it had previously.
So no checks are removed, they just have a different name.
I hope that's ok and no checks are referred to by name somewhere.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated.
• Any changes are noted in the CHANGELOG.md for affected package
• The version bounds in .cabal files are updated
• CI passes
• Self-reviewed the diff