arch-nspawn: Set machine name

Recent changes to systemd-nspawn have it take the machine name from
the chroot dir name, which isn't unique enough for our setup.

arch-nspawn.in

@@ -96,4 +96,4 @@ fi
 build_mount_args
 copy_hostconf
 
-exec systemd-nspawn -D "$working_dir" "${mount_args[@]}" "$@"
+exec systemd-nspawn -D "$working_dir" --machine "${working_dir//\//-}" "${mount_args[@]}" "$@"