native-xtools: Override proper NXBDESTDIR.

The new native-xtools uses 'make toolchain' so overriding DESTDIR
as a make argument may interfere with WORLDTMP handling.
The target also does a 'mkdir -p ${NXBDESTDIR}/usr', so we should
be modifying that rather than DESTDIR.

Note this causes the native-xtools binaries to be installed in
NANO_WORLDDIR/usr NANO_WORLDDIR/bin rather than NANO_WORLDDIR/nxb-bin/usr
and NANO_WORLDDIR/nxb-bin/bin. This was the case before this change
as well.

MFC after: 2 weeks
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D12782