explicitly drop pr approver workflow permissions

Drop all permissions on top-level. We're already giving out
actions: write on the job level.

Signed-off-by: Tuomo Tanskanen <tuomo.tanskanen@est.tech>

.github/workflows/pr-gh-workflow-approve.yaml

@@ -8,6 +8,8 @@ on:
   pull_request_target:
     types: [opened, edited, reopened, synchronize]
 
+permissions: {}
+
 jobs:
   approve:
     name: Approve on ok-to-test