ci: add flutter-builder checkout smoke workflow (act_runner 0.4.1 verification)

.gitea/workflows/flutter-runner-smoke.yml

@@ -0,0 +1,14 @@
+name: flutter-runner-checkout-smoke
+on:
+  workflow_dispatch:
+
+jobs:
+  checkout-smoke:
+    runs-on: flutter-builder
+    steps:
+      - uses: actions/checkout@v4
+      - name: Prove checkout worked
+        run: |
+          echo "checkout OK"
+          ls -la
+          git log -1 --oneline