Strictly checking pull-request author

Author: hsbt

.github/workflows/dependabot_automerge.yml

@@ -6,7 +6,7 @@ on:
 jobs:
   automerge:
     runs-on: ubuntu-latest
-    if: ${{ github.actor == 'dependabot[bot]' }}
+    if: ${{ github.event.pull_request.user.login == 'dependabot[bot]' }}
     steps:
       - name: Dependabot metadata
         uses: dependabot/fetch-metadata@v2