trigger workflow runs on bor's "staging" branch

I think:
trying is for bors try
staging is for bors r+

.github/workflows/ci.yml

@@ -2,9 +2,9 @@ name: CI
 
 on:
   push:
-    branches: [ master, trying ]
+    branches: [ master, trying, staging ]
   pull_request:
-    branches: [ master, trying ]
+    branches: [ master, trying, staging ]
   # Allows you to run this workflow manually from the Actions tab
   workflow_dispatch: