Add scripts directory to Python search directories automatically and avoid using PYTHONPATH

[wyli]

[KumoLiu]

I think we have already added the scripts directory automatically in the bundle, not sure what this means here.
https://github.com/Project-MONAI/model-zoo/blob/e6ecc282d5490f1b66ccd16948b94cd5590152e4/models/pathology_nuclei_classification/configs/evaluate.json#L69