pkgs/by-name: Enable gradual migration checks and add run-local.sh

[infinisil]

Description of changes

After #272395 extended the nixpkgs-check-by-name tool to have gradual migration checking, aka ratchet checks, this PR now enables them by passing the --base flag to the tool.

Furthermore, this is a really good opportunity to make the CI check runnable locally, solving #266931 as well as possible, replacing #266937.
Fundamentally it's impossible to make it fully reproducible because the base branch fetching/merging can't happen in the same way, but this PR makes everything be shared that can be shared between CI and the local run.

It's not perfect, but better than nothing.

Things done

• Tested in CI: Introduce empty arg in a PR tweag/nixpkgs#81
• Tested locally:

  $ ./pkgs/test/nixpkgs-check-by-name/scripts/run-local.sh by-name-reproducible https://github.com/tweag/nixpkgs.git
  Using HEAD commit cf06cb1cd4cf0f0e92fa3bd3ce894c1255fae8a1
  Creating Git worktree for the HEAD commit in /tmp/tmp.osLpsrjqvl/merged.. Done
  Fetching base branch by-name-reproducible to compare against.. cc0fe3b4b9f268e53fd0c1ce706f91a8d6af1373
  Creating Git worktree for the base branch in /tmp/tmp.osLpsrjqvl/base.. Done
  Merging base branch into the HEAD commit in /tmp/tmp.osLpsrjqvl/merged.. cf06cb1cd4cf0f0e92fa3bd3ce894c1255fae8a1
  Determining the channel to use for PR base branch by-name-reproducible.. nixos-unstable
  Fetching latest version of channel nixos-unstable.. /nix/store/k87dn8r5rr9nsa297my0h3z02hx62mnq-source
  Git revision of channel nixos-unstable is 91a00709aebb3602f172a0bf47ba1ef013e34835
  Fetching the prebuilt version of nixpkgs-check-by-name.. /nix/store/381kv4kh7sx2a43w8vcbqcy67msvyfzq-nixpkgs-check-by-name
  Running nixpkgs-check-by-name..
  pkgs.vcpkg-tool: This attribute is manually defined (most likely in pkgs/top-level/all-packages.nix), which is only allowed if the definition is of the form `pkgs.callPackage pkgs/by-name/vc/vcpkg-tool/package.nix { ... }` with a non-empty second argument.
  Validation failed, see above errors
  Cleaning up.. Done
  

• Updated contributor docs and code comments
• Clean commit history

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[infinisil]

Thanks for the detailed review @l0b0! I now squashed all the script improvements into e130ee3, adding you as a co-author :)

[infinisil]

I'll merge this, because:

• I consider pkgs/by-name: Enable gradual migration checks and add run-local.sh #274591 (comment) resolved, and it's a minor comment anyways, and we can always improve this as we go
• I tested the workflow (Introduce empty arg in a PR tweag/nixpkgs#81) and script again, all still works
• We don't need to wait for ofborg because this doesn't touch any packages
• I'll be watching CI a bit for the next hours to make sure everything goes smoothly

This allows me to proceed with finalising the next step for RFC 140:

• [RFC 140 2a] pkgs/by-name: Enforce for new packages #275539