github: new attempt at Dependabot auto-merge

This is an updated version of the workflow deleted in f5fc3c3,
trying the advice from
cli/cli#6695 (comment).

.github/workflows/dependabot.yml

@@ -0,0 +1,21 @@
+name: build
+
+on:
+  pull_request:
+
+permissions: read-all
+
+jobs:
+  dependabot-auto-merge:
+    name: 'Dependabot auto-merge'
+    permissions:
+      contents: write
+      pull-requests: write
+    runs-on: ubuntu-latest
+    if: ${{ github.actor == 'dependabot[bot]' }}
+    steps:
+    - name: Enable auto-merge for Dependabot PRs
+      run: gh pr merge --auto --rebase "$PR_URL"
+      env:
+        PR_URL: ${{github.event.pull_request.html_url}}
+        GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}