arch-nspawn: preserve systemd-nspawn's stderr

It is passed down to the container when stdin/stdout is not a tty; this
is the case when archbuild's output is redirected to a pipe or file.

arch-nspawn.in

@@ -96,7 +96,7 @@ copy_hostconf
 
 eval $(grep '^CARCH=' "$working_dir/etc/makepkg.conf")
 
-exec ${CARCH:+setarch "$CARCH"} systemd-nspawn 2>/dev/null \
+exec ${CARCH:+setarch "$CARCH"} systemd-nspawn -q \
 	-D "$working_dir" \
 	--register=no \
 	"${mount_args[@]}" \