ci/nixpkgs-vet: use the get-merge-commit workflow

.github/workflows/nixpkgs-vet.yml

@@ -19,46 +19,34 @@ permissions: {}
 # There is a feature request for suppressing notifications on concurrency-canceled runs: https://github.com/orgs/community/discussions/13015
 
 jobs:
+  get-merge-commit:
+    uses: ./.github/workflows/get-merge-commit.yml
+
   check:
     name: nixpkgs-vet
     # This needs to be x86_64-linux, because we depend on the tooling being pre-built in the GitHub releases.
     runs-on: ubuntu-latest
     # This should take 1 minute at most, but let's be generous. The default of 6 hours is definitely too long.
     timeout-minutes: 10
+    needs: get-merge-commit
     steps:
-      # This checks out the base branch because of pull_request_target
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
-        with:
-          path: base
-          sparse-checkout: ci
-      - name: Resolving the merge commit
-        env:
-          GH_TOKEN: ${{ github.token }}
-        run: |
-          if mergedSha=$(base/ci/get-merge-commit.sh ${{ github.repository }} ${{ github.event.number }}); then
-            echo "Checking the merge commit $mergedSha"
-            echo "mergedSha=$mergedSha" >> "$GITHUB_ENV"
-          else
-            echo "Skipping the rest..."
-          fi
-          rm -rf base
-      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
-        if: env.mergedSha
+        if: needs.get-merge-commit.outputs.mergedSha
         with:
           # pull_request_target checks out the base branch by default
-          ref: ${{ env.mergedSha }}
+          ref: ${{ needs.get-merge-commit.outputs.mergedSha }}
           # Fetches the merge commit and its parents
           fetch-depth: 2
       - name: Checking out base branch
-        if: env.mergedSha
+        if: needs.get-merge-commit.outputs.mergedSha
         run: |
           base=$(mktemp -d)
           git worktree add "$base" "$(git rev-parse HEAD^1)"
           echo "base=$base" >> "$GITHUB_ENV"
       - uses: cachix/install-nix-action@08dcb3a5e62fa31e2da3d490afc4176ef55ecd72 # v30
-        if: env.mergedSha
+        if: needs.get-merge-commit.outputs.mergedSha
       - name: Fetching the pinned tool
-        if: env.mergedSha
+        if: needs.get-merge-commit.outputs.mergedSha
         # Update the pinned version using ci/nixpkgs-vet/update-pinned-tool.sh
         run: |
           # The pinned version of the tooling to use.
@@ -71,7 +59,7 @@ jobs:
           # Adds a result symlink as a GC root.
           nix-store --realise "$toolPath" --add-root result
       - name: Running nixpkgs-vet
-        if: env.mergedSha
+        if: needs.get-merge-commit.outputs.mergedSha
         env:
           # Force terminal colors to be enabled. The library that `nixpkgs-vet` uses respects https://bixense.com/clicolors/
           CLICOLOR_FORCE: 1