Fix root referenced before assignment

[glenn-jocher]

Fix for bug introduced by #4919 discovered on VOC autodownload:

python train.py --data VOC.yaml

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Updated dataset autodownload and extraction process in check_dataset function.

📊 Key Changes

• Ensured the root directory for unzipping datasets is defined before downloading the .zip file.
• Improved user feedback by adding a newline character to the dataset autodownload success/failure message.

🎯 Purpose & Impact

• These changes aim to make the dataset downloading process more robust and user-friendly.
• Users will experience a smoother autodownload process with clearer communication on success or failure status. This can be particularly helpful when setting up datasets for the first time or in automated scripts. 🔄