fix(ci): split up checkout steps for push vs. pull request (#3911)

.github/workflows/shellcheck.yml

@@ -20,7 +20,15 @@ jobs:
     container:
       image: docker.io/ubuntu:24.04
     steps:
-      - uses: actions/checkout@v4
+      - name: Check out pull request code
+        uses: actions/checkout@v4
+        if: github.event_name == 'pull_request'
+        with:
+          repository: ${{ github.event.pull_request.head.repo.full_name }}
+
+      - name: Check out repository code
+        uses: actions/checkout@v4
+        if: github.event_name == 'push'
 
       - name: Install shellcheck
         run: apt update && apt install -y shellcheck bash && shellcheck --version