[Action] Use pull_request_target (microsoft#6803)

We should use the pull_request_target option here so that the PR runs
from the pipeline in the target rather than the PR source branch. This
allows the action to run with reduced security implications.

Author: llvm-beanz

.github/workflows/pr-description-checker.yml

@@ -1,6 +1,6 @@
 name: 'PR description checker'
 on:
-  pull_request:
+  pull_request_target:
     types:
       - opened
       - edited