Fix dependabot automerge.

The important fix is GITHUB_TOKEN -> GH_TOKEN. Despite the gh CLI
allowing either, apparently in the context of Github Actions
GITHUB_TOKEN is not allowed.

.github/workflows/dependabot-automerge.yml

@@ -0,0 +1,12 @@
+name: Dependabot auto-merge
+on: pull_request
+
+jobs:
+  dependabot:
+    runs-on: ubuntu-latest
+    if: ${{ github.actor == 'dependabot[bot]' }}
+      - name: Approve & enable auto-merge for Dependabot PRs
+        run: gh pr review --approve -b "Auto-approving dependabot PR." "$PR_URL" && gh pr merge --auto --squash "$PR_URL"
+        env:
+          PR_URL: ${{ github.event.pull_request.html_url }}
+          GH_TOKEN: ${{ secrets.DIVVIUP_GITHUB_AUTOMATION_DEPENDABOT_APPROVER_PAT }}