Set USBGPU if eGPU is detected

[korhojoa]

Add USBGPU environment variable setting if eGPU is detected.

This would let usbgpu be set automatically (which would allow easier testing) if the device is present when openpilot is starting.

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[korhojoa]

from discord:

[21:11]adeeb: lsusb can hang, this should not get merged as is
...
we should just get rid of the environment variable