ci: limit the number of concurrent builds

.github/workflows/check.yaml

@@ -346,6 +346,13 @@ jobs:
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v1
+        with:
+          config-inline: |
+            [registry."docker.io"]
+              mirrors = ["mirror.gcr.io"]
+
+            [worker.oci]
+              max-parallelism = 2
 
       - name: Login to GitHub Container Registry
         if: github.event_name != 'pull_request'