update greetings.yml with pull_request_target (#677)

Should resolve "Resource not accessible by integration" error message.

https://github.com/actions/first-interaction/issues/10#issuecomment-670968624

.github/workflows/greetings.yml

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