Fix PR welcome action

Was failing because GITHUB_TOKEN on regular `pull_request` events have
read-only access (for security). Change to `pull_request_target` allows
the action to run with full permissions (since it only uses workflows
from the target branch).

Author: dopplershift

.github/workflows/pr_welcome.yml

@@ -1,6 +1,6 @@
 name: PR Greetings
 
-on: [pull_request]
+on: [pull_request_target]
 
 jobs:
   greeting: