nixos/containers: Create an empty machine-id file

Since systemd version 230, it is required to have a machine-id file
prior to the startup of the container. If the file is empty, a transient
machine ID is generated by systemd-nspawn.

See systemd/systemd#3014 for more details on the matter.

This unbreaks all of the containers-* NixOS tests.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @edolstra
Closes: #15808