Cut down the evaluation overhead of `build-packages` about cardano-haskell-packages

[andreabedini]

nix-eval-jobs seems to be able to do the evaluation in parallel. It would spit out a json file we can read back into GHA. Something like this:

  eval-jobs:
    outputs:
      matrix: ${{ steps.set-matrix.outputs.matrix }}
    steps:
    - run: |
        nix-eval-jobs --flake path:$PWD#packages.x86_64-linux.allPackages --override-input CHaP path:_repo > jobs.json
    - id: set-matrix
      run: |
        echo "::set-output name=matrix::$(jq @text < dist-newstyle/cache/plan.json)" # maybe with some json wrangling
  build-packages:
    needs: eval-jobs
    strategy:
      matrix: ${{fromJson(needs.eval-jobs.outputs.matrix)}}

[michaelpj]

Hmm, interesting. Hopefully it'll use remote builders for the IFD still, but that could be a big improvement.

I'm unsure if we want to generate a matrix: probably we do if it's for the smoke test or new packages, but we wouldn't if we decided to try and build everything since matrixes are limited to 256 jobs (hah). And also the real parallelism will be happening on nixbuild.net, we'll just be occupying |matrix| runners sitting there waiting for builds to finish!

Also it doesn't yet support --override-input. Maybe we can just do --update-input beforehand instead.