prepare for garnix

[dermetfan]

• 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.

The checks output was previously empty.

Pre-submit checklist:

• Branch
  • Tests are provided (if possible)
  • Commit sequence broadly makes sense
  • Key commits have useful messages
  • Changelog fragments have been written (if appropriate)
  • Relevant tickets are mentioned in commit messages
  • Formatting, PNG optimization, etc. are updated
• PR
  • (For external contributions) Corresponding issue exists and is linked in the description
  • Targeting master unless this is a cherry-pick backport
  • Self-reviewed the diff
  • Useful pull request description
  • Reviewer requested