ci: Fix permissions for external contributors PRs (for real?) (#12776)

Another day, another try to fix this!

Now it failed like this
https://github.com/getsentry/sentry-javascript/actions/runs/9804811101/job/27073251966,
hopefully this change fixes this... we'll see!

Author: mydea

.github/workflows/external-contributors.yml

@@ -9,8 +9,12 @@ on:
 jobs:
   external_contributor:
     name: External Contributors
+    permissions:
+      pull-requests: write
+      contents: write
     runs-on: ubuntu-20.04
     if: |
+      github.event.pull_request.merged == true &&
       github.event.pull_request.author_association != 'COLLABORATOR'
       && github.event.pull_request.author_association != 'MEMBER'
       && github.event.pull_request.author_association != 'OWNER'