mpy-cross STATIC_BUILD not working

#1013 and #2551 added changes to `mpy-cross` to do static builds. Somewhere along the line, these changes were lost, and the mpy-cross executables we are now building are not static. Also see #2551. Fix this.

Looks like this was lost here: https://github.com/adafruit/circuitpython/commit/22a44c6003d9d1d1a230ca618e7a885b4e2ba6e5, in #8481.