From commit cf6e6f14d3, (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.
75e4976dde