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

.github/workflows/build-storybook.yml

@@ -11,8 +11,15 @@ jobs:
     if: github.repository == 'owncast/owncast'
 
     steps:
-      - name: Checkout
+      - 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: Cache node modules
         uses: actions/cache@v4