nixos/containers: Create an empty machine-id file
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 <ee1aa092358634f9c53f01b5a783726c9e21b35a@redmoonstudios.org>
Cc: da39a3ee5e6b4b0d3255bfef95601890afd80709@edolstra
Closes: #15808

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