Try adjusting permissions to label (#2012)

Context: https://github.com/carbon-language/carbon-lang/runs/7811528469?check_suite_focus=true

Permissions seem to be different from when I was testing, seeing if this is enough.

.github/workflows/proposal_ready.yaml

@@ -10,6 +10,9 @@ on:
     types:
       - ready_for_review
 
+permissions:
+  pull-requests: write
+
 jobs:
   proposal_ready:
     if: contains(github.event.pull_request.labels.*.name, 'proposal')