workflows/eval: fix actionlint

mergedSha is available from needs.get-merge-commit, not needs.attrs.
Actionlint rightfully complains about that.

The code still works as expected because nixpkgs/ is checked out at
mergedSha, so the diff will be between mergedSha and baseSha.

.github/workflows/eval.yml

@@ -203,7 +203,7 @@ jobs:
         if: steps.baseRunId.outputs.baseRunId
         run: |
           git -C nixpkgs worktree add ../base ${{ needs.attrs.outputs.baseSha }}
-          git -C nixpkgs diff --name-only ${{ needs.attrs.outputs.baseSha }} ${{ needs.attrs.outputs.mergedSha }} \
+          git -C nixpkgs diff --name-only ${{ needs.attrs.outputs.baseSha }} \
             | jq --raw-input --slurp 'split("\n")[:-1]' > touched-files.json
 
           # Use the base branch to get accurate maintainer info