workflows/clean-up-closed-prs: don't try to delete branch if PR is fr…

…om fork

Otherwise you can delete a branch from this repo with a PR opened from
a fork's branch with the same name.

.github/workflows/clean-up-closed-prs.yml

@@ -57,6 +57,7 @@ jobs:
   delete-branch:
     if: >
       github.repository_owner == 'Homebrew' &&
+      github.event.pull_request.head.repo.full_name == github.repository &&
       !github.event.pull_request.merged
     runs-on: ubuntu-latest
     # Ignore errors as branch may already be deleted