Add workflow_dispatch as trigger for PR validation action

Cannon pointed out this lets us re-run the action after linking an issue,
since there's no trigger for "connected" and "disconnected" events. Best
thing to do is still to link issues before the PR is officially opened, but
that shouldn't be the only way to do things!

.github/workflows/enforce-PR-labelling.yml

@@ -4,6 +4,7 @@ on:
   pull_request:
     # one limitation here is that there's no trigger to re-run any time we "connect" or "disconnect" an issue
     types: [opened, edited, labeled, unlabeled, synchronize]
+  workflow_dispatch:
 
 jobs:
   validate-pr: