npins: remove explicit dependency on nix

from what i can see other packages that depend on nix don't seem to add
it to the PATH but rather assume you have nix already in your PATH.

e.g.
https://github.com/NixOS/nixpkgs/blob/20075955deac2583bb12f07151c2df830ef346b4/pkgs/by-name/nv/nvd/package.nix
https://github.com/NixOS/nixpkgs/blob/20075955deac2583bb12f07151c2df830ef346b4/pkgs/by-name/li/lix-diff/package.nix
https://github.com/NixOS/nixpkgs/blob/20075955deac2583bb12f07151c2df830ef346b4/pkgs/tools/package-management/nix-prefetch-scripts/default.nix#L49

pkgs/by-name/np/npins/package.nix

···
   makeWrapper,
 
   # runtime dependencies
-  nix, # for nix-prefetch-url
   nix-prefetch-git,
   git, # for git ls-remote
 }:
 
 let
   runtimePath = lib.makeBinPath [
-    nix
     nix-prefetch-git
     git
   ];