Add repository sanity check to dependabot workflow

It's safer this way

Author: rubensf

.github/workflows/dependabot.yaml

@@ -7,7 +7,7 @@ permissions:
   pull-requests: read
 jobs:
   build:
-    if: ${{ github.actor == 'dependabot[bot]' }}
+    if: ${{ github.actor == 'dependabot[bot]' && github.event.pull_request.head.repo.full_name == github.repository }}
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3