update actions cache to v4

Author: sosey

.github/workflows/build-book.yml

@@ -31,7 +31,7 @@ jobs:
       run: echo "::set-output name=dir::$(pip cache dir)"
 
     - name: Cache dependencies
-      uses: actions/cache@v2
+      uses: actions/cache@v4
       with:
         path: ${{ steps.pip-cache.outputs.dir }}
         key: ${{ runner.os }}-pip-${{ hashFiles('**/requirements.txt') }}