Start the migration from CircleCI to GitHub Actions with migrating the lint
job. Used the structure from @robandpdx to setup Nix and use a GitHub key.
Restructured the check from `nix flake check` to
`nix fmt; git diff --exit-code` so we get a full patch again.
Test plan:
- Submitted this PR with a formatting error. CI failed. Submitted without and
it passed.
Co-authored-by: Rob Anderson <robandpdx@github.com>