Use YOLOv5n for CI testing

[glenn-jocher]

Faster tests!

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Update to CI testing configuration to test a different model variant in the YOLOv5 repository.

📊 Key Changes

• Changed the model used in continuous integration (CI) testing from 'yolov5s' to 'yolov5n'.

🎯 Purpose & Impact

• The purpose is to ensure the smaller, newer 'yolov5n' model is robust and functions as expected within the testing framework, showcasing continuous maintenance and quality assurance.
• The impact is a potential shift in the testing focus from a slightly larger model ('yolov5s') to the smallest model ('yolov5n'), which might be emphasizing the performance and efficiency of the lightweight model in the YOLOv5 suite. It helps ensure that updates to the YOLOv5 codebase do not break functionality for this particular model variant.

[glenn-jocher]

/rebase