Use bundler-cache

This commit updates the nightly unit test workflow to use the setup-ruby
action's built-in bundler-cache job to install gems.

See also: puppetlabs/puppet@86820ee

Author: mhashizume

.github/workflows/unit_tests_with_released_puppet_gem.yaml

@@ -34,13 +34,7 @@ jobs:
         uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}
-
-      - name: Prepare testing environment with bundler
-        run: |
-          git config --global core.longpaths true
-          bundle config set system 'true'
-          bundle config set --local without 'release'
-          bundle update --jobs 4 --retry 3
+          bundler-cache: true
 
       - name: Run unit tests
         run: bundle exec rake parallel_spec