/var/lib/nixos is used by update-users-groups.pl in the activation
script for storing uid/gid mappings. If this has its own mountpoint
(as is the case in some setups with fine-grained bind mounts pointing
into persistent storage), the mappings are written to /var/lib, /var,
or /. These may be backed by a tmpfs or (otherwise ephemeral storage),
resulting in the mappings not persisting between reboots.
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