workflows/eval: fix comparison with merge conflicts

In PRs with multiple commits and merge conflicts the logic "targetSha ==
immediate parent of mergedSha" doesn't hold anymore. The head and base
commits of the PR's branch have some commits inbetween them, instead.

Before this change, we'd get a "fatal: invalid reference" on the
"worktree add". Now, not anymore, because we fetch the right commit
directly.

(cherry picked from commit cd9a22d753)

.github/workflows/eval.yml

@@ -101,7 +101,6 @@ jobs:
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
           ref: ${{ needs.prepare.outputs.mergedSha }}
-          fetch-depth: 2
           path: nixpkgs
 
       - name: Install Nix
@@ -168,6 +167,7 @@ jobs:
         env:
           AUTHOR_ID: ${{ github.event.pull_request.user.id }}
         run: |
+          git -C nixpkgs fetch --depth 1 origin ${{ needs.prepare.outputs.targetSha }}
           git -C nixpkgs worktree add ../target ${{ needs.prepare.outputs.targetSha }}
           git -C nixpkgs diff --name-only ${{ needs.prepare.outputs.targetSha }} \
             | jq --raw-input --slurp 'split("\n")[:-1]' > touched-files.json