Oh wow, people are watching my videos, how embarrassing
If I remember correctly this was a bit of a joke.
When NIX_PATH has a value that points to a URL, Nix will fetch that URL as a Tarball and extract it into the /nix/store. You can try this out like this:
$ NIX_PATH=nixpkgs=https://github.com/NixOS/nixpkgs/archive/master.tar.gz nix-instantiate --eval --expr "<nixpkgs>"
<nixos-config> in the code, it would be possible to use something like
nixos-rebuild -I nixos-config=https://github.com/myuser/myconfig/archive/master.tar.gz switch (where
-I extends the NIX_PATH).
But that’s the theory. Because the tarball returns a folder, it means that the configuration must be located in the repo top-level and named “default.nix”. So I don’t know how practical that is.