github: enable auto-merge on Dependabot PRs

To merge a Dependabot PR, I have to enable auto-merge (two clicks,
including one to confim) and then review and approve it. Since our
branch protections require the PR to be approved, it seems that that
should be enough. This patch adds a GitHub action that calls runs the
GitHub CLI to do that. It is based on
https://dev.to/slashgear_/how-to-automatically-merge-dependabot-pull-requests-with-github-actions--30pe

.github/workflows/dependabot.yml

@@ -0,0 +1,18 @@
+name: build
+
+on:
+  pull_request:
+
+permissions: read-all
+
+jobs:
+  dependabot-auto-merge:
+    name: 'Dependabot auto-merge'
+    runs-on: ubuntu-latest
+    if: ${{ github.actor == 'dependabot[bot]' }}
+    steps:
+    - name: Enable auto-merge for Dependabot PRs
+      run: gh pr merge --auto "$PR_URL"
+      env:
+        PR_URL: ${{github.event.pull_request.html_url}}
+        GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}