Hardening auto-merge workflow

https://docs.github.com/en/code-security/dependabot/working-with-dependabot/automating-dependabot-with-github-actions#enabling-automerge-on-a-pull-request

Author: hsbt

.github/workflows/dependabot_automerge.yml

@@ -1,14 +1,16 @@
 # from https://github.com/gofiber/swagger/blob/main/.github/workflows/dependabot_automerge.yml
 name: Dependabot auto-merge
 on:
-  pull_request_target:
+  pull_request:
+
+permissions:
+  contents: write
+  pull-requests: write
 
 jobs:
   automerge:
     runs-on: ubuntu-latest
-
-    if: ${{ github.event.pull_request.user.login == 'dependabot[bot]' }}
-
+    if: github.event.pull_request.user.login == 'dependabot[bot]' && github.repository == 'ruby/ruby'
     steps:
       - name: Dependabot metadata
         uses: dependabot/fetch-metadata@dbb049abf0d677abbd7f7eee0375145b417fdd34 # v2.2.0