There's a closed pull request on the void-packages repository

fix cross python because of change in uname wrapper
https://github.com/void-linux/void-packages/pull/20311

Description:
From commit cf6e6f1, (wrappers/uname: report correct machine type
on ethereal, 2020-02-10), uname wrapper has been updated to report
correct machine type in ethereal mode by replace its original machine
type by value reported by $XBPS_ARCH.

This effectively breaks python{,3}{,-tkinter}'s cross compile because
we reset all environment variable to configure for the host build.

Keep those environment variable.