Use pre-built mpy-cross executables

[dhalbert]

Pre-built mpy-cross executables are available at https://adafruit-circuit-python.s3.amazonaws.com/index.html?prefix=bin/mpy-cross/

Using pre-built ones would save time, particularly fetching submodules.

[tekktrik]

I can definitely make this change, that seems way better than building from scratch.