Cache bundle gem for better performance

.github/workflows/build.yaml

@@ -9,7 +9,13 @@ jobs:
 
     steps:
     - uses: actions/checkout@v1
+    - uses: actions/cache@v1
+        with:
+          path: vendor/bundle
+          key: ${{ runner.os }}-gem-${{ hashFiles('**/Gemfile.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-gem-
     - name: Install Gems
-      run: bundle install
+      run: bundle install --path vendor/bundle
     - name: Run tests
       run: bundle exec fastlane tests