Add _build to book cache

Author: finsberg

.github/workflows/build_docs.yml

@@ -25,19 +25,21 @@ jobs:
         with:
           python-version: "3.10"
 
-      - name: Install dependencies
-        run: python3 -m pip install -r requirements-docs.txt
-
       - name: Cache
         id: cache
         uses: actions/cache@v3
         with:
           path: |
             ~/.cache/pip
+            ~/_build
           key: cache_v1
           restore-keys: |
             cache_v1
 
+      - name: Install dependencies
+        run: python3 -m pip install -r requirements-docs.txt
+
+
       - name: Build docs
         run: jupyter book build .