Easy to show bundle exec rubocop output at Action

When pull requests have RuboCop offenses they need to click
"Build and run RuboCop" section and scroll about 200 lines
to skip `bundle install` output.

This change splits "Build and run RuboCop" into two parts.
then "Run RuboCop" section only shows `bundle exec rubocop --parallel`
output

.github/workflows/rubocop.yml

@@ -23,8 +23,9 @@ jobs:
         key: ${{ runner.os }}-gem-${{ hashFiles('**/Gemfile.lock') }}
         restore-keys: |
           ${{ runner.os }}-gem-
-    - name: Build and run RuboCop
+    - name: Install gems
       run: |
         bundle config path vendor/bundle
         bundle install --jobs 4 --retry 3
-        bundle exec rubocop --parallel
+    - name: Run RuboCop
+      run: bundle exec rubocop --parallel