nix-prefetch-scripts: explicitly depend on Nix

Otherwise, a call to, for example, nix-prefetch-git, in a nix-shell --pure or
nix-build phase would result in an error such as:

    Switched to a new branch 'fetchgit'
    removing `.git'...
    /nix/store/7qvjji7kbd033nsyxlpiiiam61a44yz9-nix-prefetch-git/bin/.nix-prefetch-git-wrapped: line 374: nix-hash: command not found