(root)# apt install nix-bin

(user)$ cd /some/where
(user)$ git clone https://github.com/nixos/nixpkgs
(user)$ cd nixpkgs
(user)$ nix-build . -A gnucap-full
(user)$ cd
[..]

(root)# mount -o bind ~user/.local/share/nix/root/nix/ /nix # is this needed?

(user)$ nix-shell -p gnucap-full -I nixpkgs=/some/where/nixpkgs
[nix-shell:~]$ gnucap
[..]
gnucap> _