This cropped up, because I have a set-up where my work username is
different to my home desktop username, and I am using a parameterized
config for both, so I have something akin to
config.users.users.default-user = ...;
and using
config.users.users.default-user.{name, home}
in certain places to cope with this. Noticed my home-manager bought in
packages (which use the users.users.<name>.packages hence NixOS issue
not home-manager) weren't present.