Fix OMP_NUM_THREADS=1 for macOS

[glenn-jocher]

Resolves #8623

🛠️ PR Summary

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🌟 Summary

Improvement in thread management for macOS within the YOLOv5 codebase.

📊 Key Changes

• Modified the environment variable setting for OMP_NUM_THREADS based on the operating system.
• For macOS (darwin), this variable is hardcoded to '1', indicating a single thread.
• For other operating systems, it continues to follow the NUM_THREADS setting.

🎯 Purpose & Impact

• Enhances compatibility and stability of YOLOv5 on macOS by preventing multithreading issues specific to the platform.
• Likely to improve the user experience on macOS by eliminating potential crashes or performance bottlenecks due to threading conflicts.
• Users on other platforms should see no change in performance or behavior, maintaining the expected multithreading capabilities.