TZ
вот в мануале запись "If nix is already installed on the system i.e. a self-compiled nix itself can also create a nix store in an alternative user-writable locations"
и приведён пример
The following command will create a nix store in ~/my-nix and spawn a shell, where ~/my-nix be mounted to /nix.
$ nix run --store ~/my-nix nixpkgs.nix nixpkgs.bashInteractive