Centralize user_config_dir() decision making

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhancements to user configuration directory management in Ultralytics' YOLOv5 codebase.

📊 Key Changes

• user_config_dir function now supports an environment variable YOLOV5_CONFIG_DIR.
• If YOLOV5_CONFIG_DIR is set, it will use this path for the user config.
• If the environment variable is not set, it falls back on OS-specific config directories.
• Environment variable support facilitates more flexible configurations and deployments.

🎯 Purpose & Impact

• 🎨 Flexibility: Users can easily set a custom configuration directory through an environment variable.
• 🔧 Reliability: The function ensures the config path exists or is writable, falling back to /tmp if necessary.
• ☁️ Cloud-Friendly: Improvements are especially beneficial for cloud deployments (e.g., GCP, AWS Lambda) where only certain directories like /tmp may be writable.
• 📈 Enhanced User Experience: Non-expert users benefit from a system that adapts to their environment seamlessly, while power users can customize their setup more granularly.