We're no longer running this on `staging` PRs, and users doing single-package
updates have the option to omit `nixpkgs-review`, so we should only include the
PR comment section if it was actually run.
This continues to enhance the automated tests for the PR comment formatting,
so you can see what the diff looks like just by looking at the test suite!
* it isn't necessary to run cabal2nix directly (developPackage) takes
care of that.
* the cabal file is committed, so it only needs to be updated when
adding dependencies or new files
