Use default Bundler for ruby-head CI (#523)

Author: k0kubun

.github/workflows/test.yml

@@ -33,6 +33,7 @@ jobs:
         uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}
+          bundler: ${{ matrix.ruby == 'head' && 'default' || 'Gemfile.lock' }}
           bundler-cache: true
 
       - name: Run tests