Update README.md

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Streamlined the PR Summary GitHub Action script fetching method.

📊 Key Changes

• The original inline script execution from utils/run_pr_summary.py has been replaced.
• Now the script is fetched directly from the GitHub repository using curl before execution.

🎯 Purpose & Impact

• Enhanced Maintainability: Fetching the latest version of the script ensures that the action is always up-to-date with the repository's latest changes.
• Robust Execution: Separating the download and execution steps may reduce potential errors within the action runtime.
• User Transparency: Users can see a clear reference to the source of the script, increasing trust in the action's functionality.