CI: Provide `write` permission for `pull-requests`

The "recent base" check attempts to remove a label from the PR being
checked, which uses the `issues` API. But a `write` permission for the
`issues` API appears to be insufficient.

.github/workflows/checks.yml

@@ -5,6 +5,7 @@ on: pull_request_target
 permissions:
   contents: read
   issues: write
+  pull-requests: write
 
 jobs:
   recent-base: