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

tools/tools/nanobsd/defaults.sh

@@ -438,7 +438,7 @@ native_xtools ( ) (
 	nano_make_install_env
 	set -o xtrace
 	cd "${NANO_SRC}"
-	${NANO_MAKE} native-xtools DESTDIR="${NANO_WORLDDIR}"
+	${NANO_MAKE} native-xtools NXBDESTDIR="${NANO_WORLDDIR}"
 
 	) > ${NANO_LOG}/_.native_xtools 2>&1
 )