mdbookをインストールするコマンドを追加

Author: polyester-CTRL

.github/workflows/ci.yml

@@ -83,8 +83,20 @@ jobs:
           ./atcoder-problems-frontend/node_modules
         key: ${{ runner.os }}-cargo-${{ hashFiles('atcoder-problems-frontend/yarn.lock') }}
 
+    - name: Cache cargo registry
+      uses: actions/cache@v2
+      with:
+        path: |
+          ~/.cargo/git
+          ~/.cargo/registry/cache
+          ~/.cargo/registry/index
+          ./atcoder-problems-backend/target
+        key: ${{ runner.os }}-cargo-${{ hashFiles('atcoder-problems-backend/Cargo.lock') }}
+
     - name: Install dependencies
       run: yarn
+    - name: Setup mdBook
+      run: cargo install mdbook
     - name: build
       run: yarn build
     - name: test