Update checkout action to v4

Author: sosey

.github/workflows/build-book.yml

@@ -15,7 +15,7 @@ jobs:
   build-test-book:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
 
     - name: Setup Python
       uses: actions/setup-python@v2