Added pull_request event type to label actions workflow triggers

refs https://github.com/TryGhost/Toolbox/issues/381

- we want to start collecting events from pull requests so we can label
  and reply upon certain events
- this adds the `closed` and `labeled` events to the workflow triggers

.github/workflows/label-actions.yml

@@ -4,6 +4,8 @@ on:
   workflow_dispatch:
   issues:
     types: [opened, closed, labeled]
+  pull_request:
+    types: [closed, labeled]
   schedule:
     - cron:  '0 * * * *'