Mitigate pull_request_target privilege escalation

Hotfix — replaces pull_request_target with pull_request to stop
granting write permissions and secrets to fork PRs. Some workflows
will break; can be fixed properly later.

.github/workflows/project-automation.yml

@@ -4,7 +4,7 @@ on:
   push:
     branches:
       - master
-  pull_request_target:
+  pull_request:
   issue_comment:
 
 permissions: {}