workflows/eval: Minor fixes, ensure the correct commit is checked out

[infinisil]

• env.mergedSha is empty, so it checked out the master version by default
• The process step used needs.attrs.outputs.mergedSha, but apparently that's empty unless attrs is declared as a needs, even though outputs implicitly depends on attrs

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

---

This work is funded by Tweag and Antithesis ✨

Add a 👍 reaction to pull requests you find important.

[azuwis]

Actually actionlint found this:

.github/workflows/eval.yml:118:20: property "attrs" is not defined in object type {outpaths: {outputs: {}; result: string}} [expression]
    |
118 |           ref: ${{ needs.attrs.outputs.mergedSha }}
    |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

But i was not quite familiar with cross job variable passing, and ignored it.

Maybe we can add an actionlint workflow?

[Mic92]

#358087