Add self package to flake.nix, nix fixes, fmt

[KaiSforza]

• Added a default/clawscli package that builds this project. Note,
  vendorHash has to be set when the dependencies change.
• Uses the same flags as in the .goreleaser.yml
• Uses self for automatic version options using the hash of the current
  git revision. Uses the dirtyShortRev if it is available so that
  builds and versions show that there are changes to the working copy.
  Nix already takes care of making the output different, so no bumping
  versions are necessary to do nix build ...
• Formatted the flake.nix using nixfmt.
• Used built-in stdenv.targetPlatform.node variables for fetching
  indexion, set meta.platforms to limit where it can run.
• Added nix info to the README.
• Does replacement for /bin/echo in tests
• Uses a temporary home directory in tests for config tests
• Added nix checks into a separate Github actions job
• Ignore nix output results

[yimsk]

Thanks for adding the Nix package and installation instructions.

It would be good to add a workflow that builds the Nix package itself. The current workflows run the Go build/tests and nix develop, but they do not run nix build .#clawscli. As a result, if vendorHash becomes stale after a dependency change, CI could remain green while installation through Nix is broken.

Could we add nix build .#clawscli after the Nix setup in the Integration Test workflow and, if practical, also run the generated claws --version as a smoke test?

I also left an inline comment about limiting the package output to the claws binary.

For the skipped tests, it would be better in the long term to make them runnable in the Nix environment. However, that can be handled as a follow-up, and I do not think the current skip list needs to block this PR.

[KaiSforza]

It would be good to add a workflow that builds the Nix package itself. The current workflows run the Go build/tests and nix develop, but they do not run nix build .#clawscli. As a result, if vendorHash becomes stale after a dependency change, CI could remain green while installation through Nix is broken.

Could we add nix build .#clawscli after the Nix setup in the Integration Test workflow and, if practical, also run the generated claws --version as a smoke test?

I added it as a separate job, since none of the go setup needs to happen outside of Nix anyway.

[yimsk]

Thanks! That approach works for me.

[KaiSforza]

Alright, some changes:

• new package output, gen-imports so this can be run from nix if it's being used for development
• Excluded scripts/gen-imports from the normal derivation's output
• Fixed tests that reference /bin/echo using substituteInPlace
• Added result and result-* to .gitignore