Support homebrew libusb on macOS

[cdwilson]

For macOS running on Apple Silicon, installing libusb via homebrew (brew install libusb) installs the libraries to /opt/homebrew/lib/. Older versions of python (e.g. 3.7 installed via pyenv) won't be able to find the homebrew libusb libraries. When trying to use a device like the GreatFET One that uses pyusb, bin/detect.py will throw an error like this:

❯ bin/detect.py 
Traceback (most recent call last):
  File "bin/detect.py", line 35, in <module>
    print("Chip id: ", detector.chip.id)
  File "/Users/chris/Projects/Adafruit_Python_PlatformDetect/adafruit_platformdetect/chip.py", line 138, in id
    if usb.core.find(idVendor=0x1D50, idProduct=0x60E6) is not None:
  File "/Users/chris/.pyenv/versions/test-platformdetect/lib/python3.7/site-packages/usb/core.py", line 1309, in find
    raise NoBackendError('No backend available')
usb.core.NoBackendError: No backend available

This PR just adds the homebrew libusb path to the macOS DYLD fallback library search path:

❯ bin/detect.py 
Chip id:  LPC4330
Board id:  GREATFET_ONE
...
Is this an OS environment variable special case? True

[makermelissa]

Thanks