fix: use pull_request_target in close-llm-pr.yml (#26232)

So that it actually has write permissions; this should be safe as it doesn't use any external input.

.github/workflows/close-llm-pr.yml

@@ -1,7 +1,7 @@
 name: Close LLM-generated PRs
 
 on:
-  pull_request:
+  pull_request_target:
     types: [labeled]
 
 permissions: {}