dependabot-automerge.yml: add GH_TOKEN

GitHub Actions sets `GITHUB_TOKEN` but `gh` needs `GH_TOKEN`

Author: JasonGross

.github/workflows/dependabot-automerge.yml

@@ -10,6 +10,7 @@ jobs:
     if: ${{ github.actor == 'dependabot[bot]' }}
     env:
       PR_URL: ${{github.event.pull_request.html_url}}
+      GH_TOKEN: ${{ github.token }}
     steps:
       #- name: approve
       #  run: gh pr review --approve "$PR_URL"