pbuilder: Update native to USERSPACE_ARCH

Corallary to https://coral-review.googlesource.com/c/build/+/12734 -- we need to
update all calls of the "native" toolchain to use the USERSPACE_ARCH variable to
match up with the naming change.

Change-Id: Ib01faa0900195cf84051ca729d0918cb7c929b9c
1 file changed