Revert pack_generator system call changes to fix test failures

Reverted the array-form system calls back to string form to match
the test expectations in pack_generator_spec.rb.

This change (commit dabb298) was unrelated to the Shakapacker 9.3.0
upgrade and broke 3 tests:
- runs pack generation successfully in verbose mode
- runs pack generation successfully in quiet mode
- exits with error when pack generation fails

The tests expect the old string-based format:
  system "bundle exec rake react_on_rails:generate_packs"

Addresses review comment from @justin808 about unrelated changes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: justin808

lib/react_on_rails/dev/pack_generator.rb

@@ -73,9 +73,9 @@ def run_rake_task_directly(silent: false)
 
         def run_via_bundle_exec(silent: false)
           if silent
-            system("bundle", "exec", "rake", "react_on_rails:generate_packs", out: File::NULL, err: File::NULL)
+            system "bundle exec rake react_on_rails:generate_packs > /dev/null 2>&1"
           else
-            system("bundle", "exec", "rake", "react_on_rails:generate_packs")
+            system "bundle exec rake react_on_rails:generate_packs"
           end
         end
       end