fix(.github/workflows): detected GitHub workflow tokens with excessive permissions in auto-assign.yml (#4021)

Signed-off-by: Gaius <gaius.qi@gmail.com>

Author: gaius-qi

.github/workflows/auto-assign.yml

@@ -4,6 +4,9 @@ on:
   pull_request_target:
     types: [opened, reopened, ready_for_review]
 
+permissions:
+  pull-requests: write
+
 jobs:
   add-assignee:
     runs-on: ubuntu-latest