ci: switch checkout action source away from gitea.com mirror

The gitea.com checkout action mirror is timing out during workflow startup. Use actions/checkout@v4 directly so jobs do not fail before any repository logic runs.

.gitea/workflows/kubeadm-bootstrap.yml

@@ -27,7 +27,7 @@ jobs:
           fi
 
       - name: Checkout repository
-        uses: https://gitea.com/actions/checkout@v4
+        uses: actions/checkout@v4
 
       - name: Create SSH key
         run: |