allow choosing of which branch to run pr-demo-deploy on

Author: chrisjsimpson

.github/workflows/pr-demo-deploy.yml

@@ -5,6 +5,7 @@
 
 name: Deploy pr preview
 on:
+  workflow_dispatch: # Allow choosing from which branch to run action on
   pull_request:
   # (pull_request_target get fired on external contributer pull requests)
   #pull_request_target