Update auto-merge.yml (#231)

.github/workflows/auto-merge.yml

@@ -6,8 +6,13 @@ on:
 jobs:
   auto-merge:
     runs-on: ubuntu-latest
+    if: ${{ github.actor == 'dependabot[bot]' }}
     steps:
       - uses: actions/checkout@v2
+        with:
+          # Check out the pull request HEAD
+          ref: ${{ github.event.pull_request.head.sha }}
+          github-token: ${{ secrets.GITHUB_TOKEN }}
       - uses: ahmadnassri/action-dependabot-auto-merge@v2
         with:
           target: major