add rubocop to ruby workflow

Author: EmCousin

Diff for: .github/workflows/ruby.yml

@@ -36,3 +36,5 @@ jobs:
         bundler-cache: true # runs 'bundle install' and caches installed gems automatically
     - name: Run tests
       run: bundle exec rspec
+    - name: Run Rubocop
+      run: bundle exec rubocop