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

• Commits in meaningful sequence and with useful messages
• Tests added or updated when needed
• CHANGELOG.md files updated for packages with externally visible changes
• Versions updated in .cabal and CHANGELOG.md files when necessary, according to the versioning process.
• Version bounds in .cabal files updated when necessary
• Code formatted (use scripts/fourmolize.sh)
• Cabal files formatted (use scripts/cabal-format.sh)
• hie.yaml updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[lehins]

FYI. I am sure you are a ware, but Garnix is reporting "You have exhausted your monthly CI quota" on all PRs

[dermetfan]

I was not aware, thank you @lehins.

[dermetfan]

Canceled auto-merge because I just talked to the garnix folks and we're out of quota for the trial, so no point in merging this if it won't be used. I'll merge if we can get some more quota.

[lehins]

I'll merge if we can get some more quota.

Sounds good. I'll change it to draft so it doesn't get merged accidentally