GHA eval support

[wegank]

Since NixOS/nixpkgs#359704, there is an evaluation output available as an artifact in the Eval / Process (pull_request_target) check. It would be nice to reuse it to skip local evaluation.

[Mic92]

@GaetanLepage is working on this.

[GaetanLepage]

Yes, I have started to look at it and will work on it this WE.

[getchoo]

Is ./ci/eval faster than the current method used for local eval? Could be nice for running nixpkgs-review rev HEAD before pushing

[GaetanLepage]

Is ./ci/eval faster than the current method used for local eval? Could be nice for running nixpkgs-review rev HEAD before pushing

Well, there are two things that I want to add to nixpkgs-review:

1. Retrieving the eval results from the GHA (when available) to prevent doing any work locally.
2. Use the new eval code in ci/eval to perform multi-threaded evaluation locally.