Set top level token permission for roll.yaml. (#205)

Author: jianjunz

.github/workflows/roll.yaml

@@ -1,4 +1,6 @@
 name: Roll WebRTC revision
+permissions: read-all
+
 on:
   push:
     branches:
@@ -21,4 +23,4 @@ jobs:
     - run: pip install requests
     - run: python scripts/roll_webrtc.py --base_branch main --revision $GITHUB_SHA --token $OWT_BOT_TOKEN
       env:
-        OWT_BOT_TOKEN: ${{ secrets.OWT_BOT_TOKEN }}
+        OWT_BOT_TOKEN: ${{ secrets.OWT_BOT_TOKEN }}