fix(ci): automerge on pull_request_target instead of pull_request

- run automerge on pull_request_target instead of pull_request
- pull_request does not have access to repository secrets (since the CI
  runs on the pull_request code which might me malicious)
- pull_request_target instead runs on target branch code and has access
  to repository secrets
- pull_request_target is fine for automerge
- explaining [stackoverflow post](https://stackoverflow.com/a/74959635)

Signed-off-by: AtomicFS <[email protected]>

.github/workflows/bot--automerge.yml

@@ -3,7 +3,7 @@
 name: 'auto-merge'
 
 on:
-  pull_request:
+  pull_request_target:
     types: [opened, synchronize, reopened, ready_for_review]
 
 permissions: read-all