Switch to pull_request workflow trigger

We don't need workflows to run in context of main branch
(i.e. pull_request_target trigger) since the workflows don't
have access to any secrets. Using pull_request_target just
makes it more tedious to test changes to the workflows.

Author: sd109

.github/workflows/test-pr.yml

@@ -2,7 +2,7 @@
 name: Test pull request
 
 on:
-  pull_request_target:
+  pull_request:
     types:
       - opened
       - synchronize