checkout merge commit of PR, not base branch head

https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target

.github/workflows/tests.yml

@@ -12,6 +12,8 @@ jobs:
       options: --user root
     steps:
       - uses: actions/checkout@v4
+        with:
+          ref: '${{ github.event.pull_request.merge_commit_sha }}' # since event is pull_request_target
       - name: Install Packages
         run: yarn install --frozen-lockfile
       - name: Build