use `pull_request_target` to run PR block action

To avoid requiring authorization to run the PR block action for new
contributors, use `pull_request_target`. Running the workflow file from
the destination branch does not require approval.

- https://stackoverflow.com/questions/74957218/what-is-the-difference-between-pull-request-and-pull-request-target-event-in-git

.github/workflows/pr_block.yml

@@ -7,7 +7,7 @@
 
 name: PR block
 on:
-  pull_request:
+  pull_request_target:
     branches:
       - stable