test: try gem install rubocop

Author: FarhanAnjum-opti

.github/workflows/ruby.yml

@@ -37,8 +37,7 @@ jobs:
       if: matrix.ruby == '3.0.0'
       run: |
         echo "Installing rubocop 1.78.0 for Ruby 3.0.0"
-        bundle add rubocop --version 1.78.0 || true
-        bundle install
+        gem install rubocop -v 1.78.0
     - name: Run linting
       run: |
         bundle exec rubocop