fix permission issue with fix:refcache PR action (open-telemetry#3622)

Author: svrnm

.github/workflows/pr-actions.yml

@@ -94,6 +94,7 @@ jobs:
       contains(github.event.comment.body, '/fix:refcache')
     permissions:
       contents: write
+      pull-requests: write
 
     env:
       DEPTH: --depth 100 # submodule clone depth