This has only been used by Dysnomia, which has been removed from Nixpkgs
in https://github.com/NixOS/nixpkgs/pull/110799 after being broken for
more than a year.
If Dysnomia comes back, it can probably just use
/nix/var/nix/profiles/default/lib/systemd/system, or set its own systemd
flavour looking in that location via the `systemd.package`.
e373b423c7