Get NixOS services on a not-NixOS system

You still need nix though.
Also requires that the system runs systemd because NixOS systems are based on systemd.
Also assumes the system uses dpkg (Debian, Ubuntu, ...) but you can probably work around that.

Overview

You know how a NixOS configuration is a module where you enable a bunch of services and whatever.
nixos-rebuild combines this configuration module with the rest of the NixOS modules to create a system.
This system is a derivation, put into a profile somewhere and linked around the filesystem.
- Remember that profiles are in /nix/var/nix/profiles.
What if you could control this last linking step? Carefully install only the services you want, without installing a whole NixOS system.
Basically this is what this is.
Each "service" directory contains a kind of NixOS configuration, which specifies the services to enable, and build instructions for a package.
The NixOS configuration is put into a profile.
The package contains the symlinks to the profile, so that the system's package manager handles the necessary system state.
After installing a package, updates of the services are done on the profile, so reinstalling the package isn't necessary (unless there are new files).

nix run .#yggdrasil profile to print the profile where the service will be installed.
nix run .#yggdrasil update to add/update the yggdrasil service in the current user's profile directory.
sudo nix run .#yggdrasil update to do this on the root user's profile directory.
- This is necessary because the packages (see next point) link to the root user's profile directory, but if you're not using a package for a particular module then you can link it to your own user's profile and figure it out yourself.
sudo nix run .#yggdrasil history to see the profile history, sudo nix run .#yggdrasil rollback --to 123 to switch to another version.
sudo nix run .#yggdrasil help to see a summary of things you can do.
nix build .#package-yggdrasil to get a Debian/Ubuntu package containing the symlinks to the service's profile.
You'll need to manually start and stop the service, e.g. sudo systemctl start yggdrasil.service.
The service is automatically enabled though (mainly to reduce the amount of unaccounted-for system state), so you can also just reboot.
Systemd will complain but probably won't do anything else when you remove the service files of a running service. But unless it's critical networking stuff you probably want to stop them first anyway.
Remember to restart the service after updates.

Good luck.
Make a directory, say foobar.
Add a foobar/default.nix file containing the configuration module, a debianControl attribute containing text for a Debian package control file, and an installScript attribute containing the package install script to create the package.
Add "foobar" to the services in flake.nix (under line 8 at this time).
🎉

sudo dpkg-divert --package service-firewall --divert /etc/nftables.conf.divert --rename --add /etc/nftables.conf
- The firewall service here interferes with nftables in Ubuntu, but the existing nftables service is valuable so, instead of specifying a conflict in the package, the conflict is diverted.
sudo apt autoremove
- The ubuntu-advantage-tools-stub """service""" (it's unrelated to NixOS, it only meaningfully makes a package for Ubuntu to remove ads) replaces ubuntu-advantage-tools in Ubuntu, and some dependencies are left dangling as a result.