ci: Use pull_request_target to trigger gh-ph

[1] Frederick888/gh-ph@17b4944

Signed-off-by: Frederick Zhang <frederick888@tsundere.moe>

Author: Frederick888

.github/workflows/gh-ph.yml

@@ -1,7 +1,7 @@
 name: Pull request history
 
 on:
-  pull_request:
+  pull_request_target:
 
 permissions:
   contents: read
@@ -10,7 +10,6 @@ permissions:
 jobs:
   gh-ph:
     name: Add commit history to pull request description
-    if: github.event.pull_request.head.repo.full_name == github.repository
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3