Use Bundler's BUNDLE_GEMFILE feature and setup-ruby's bundler-cache feature

Author: amatsuda

.github/workflows/ci.yml

@@ -56,25 +56,16 @@ jobs:
             allow-failure: true
           - ruby: head
             gemfile: Gemfile
-            allow-failure: true
+
+    env:
+      BUNDLE_GEMFILE: ${{ matrix.gemfile }}
 
     steps:
       - uses: actions/checkout@v2
-      - uses: actions/cache@v2
-        with:
-          path: /home/runner/bundle
-          key: bundle-use-ruby-${{ matrix.ruby }}-${{ matrix.gemfile }}-gems-${{ hashFiles(matrix.gemfile) }}-${{ hashFiles('**/*.gemspec') }}
-          restore-keys: |
-            bundle-use-ruby-${{ matrix.ruby }}-${{ matrix.gemfile }}-gems-
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}
-      - name: Bundle install
-        run: |
-          gem install bundler -v 2.1.4
-          bundle config path /home/runner/bundle
-          bundle config --global gemfile ${{ matrix.gemfile }}
-          bundle install --jobs 4 --retry 3
+          bundler-cache: true
       - name: Run tests
         run: bundle exec rake