You can actually put your NixOS config anywhere. Assuming you're not using flakes, `nixos-rebuild` finds the config through NIX_PATH:

grahamc@scruffy:~/ > echo $NIX_PATH

nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos:nixos-config=/etc/nixos/configuration.nix:/nix/var/nix/profiles/per-user/root/channels

you can change the `nixos-config` value to point to somewhere else, or call it like this:`nixos-rebuild -I nixos-config=./configuration.nix`