Only do previews for successful workflows

Author: holly-cummins

.github/workflows/preview.yml

@@ -19,6 +19,7 @@ jobs:
         with:
           workflow: ${{ github.event.workflow_run.workflow_id }}
           run_id: ${{ github.event.workflow_run.id }}
+          workflow_conclusion: success
           name: site
       - name: Store PR id as variable
         id: pr