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

- `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`

(cherry picked from commit 19db54eda1)

.github/workflows/eval.yml

@@ -38,7 +38,7 @@ jobs:
         # Add this to _all_ subsequent steps to skip them
         if: steps.merged.outputs.mergedSha
         with:
-          ref: ${{ env.mergedSha }}
+          ref: ${{ steps.merged.outputs.mergedSha }}
           path: nixpkgs
 
       - name: Install Nix
@@ -104,7 +104,7 @@ jobs:
   process:
     name: Process
     runs-on: ubuntu-latest
-    needs: outpaths
+    needs: [ outpaths, attrs ]
     steps:
       - name: Download output paths and eval stats for all systems
         uses: actions/download-artifact@fa0a91b85d4f404e444e00e005971372dc801d16 # v4.1.8