Improve PNNX assets check robustness

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Improved reliability in fetching PNNX assets for different operating systems.

📊 Key Changes

• Removed the retry option from the get_github_assets function call.
• Added an assertion to ensure a URL is found for the PNNX repo assets.
• Established a fallback URL with a specific version if the asset retrieval fails.

🎯 Purpose & Impact

• Enhance Stability: By removing the retry option and adding an assertion, the process is simplified, aiming at reducing potential errors during asset fetching. 🛠️
• Better Error Handling: In cases where the PNNX assets cannot be found, a default fallback URL ensures that the operation can still proceed, albeit with a specific version. This minimizes disruptions to the user experience. 🔄
• User Confidence: The changes help ensure that users have access to the necessary assets regardless of their operating system, which is crucial for seamless functionality across different environments. 🌍✅