Restrict TRT autoinstall to Linux-only

May partially resolve concerns in #7537 (comment)

export.py

@@ -218,7 +218,8 @@ def export_engine(model, im, file, train, half, simplify, workspace=4, verbose=F
     # YOLOv5 TensorRT export https://developer.nvidia.com/tensorrt
     try:
         assert im.device.type != 'cpu', 'export running on CPU but must be on GPU, i.e. `python export.py --device 0`'
-        check_requirements(('nvidia-tensorrt',), cmds=('-U --index-url https://pypi.ngc.nvidia.com',))
+        if platform.system() == 'Linux':
+            check_requirements(('nvidia-tensorrt',), cmds=('-U --index-url https://pypi.ngc.nvidia.com',))
         import tensorrt as trt
 
         if trt.__version__[0] == '7':  # TensorRT 7 handling https://github.com/ultralytics/yolov5/issues/6012