Another option would be to look for nix-shell shebang, like

       #! /usr/bin/env nix-shell
       #! nix-shell -i runghc --packages 'haskellPackages.ghcWithPackages (ps: [ps.download-curl ps.tagsoup])'

in the init script where we determine the type in amazon-init.nix and run it with nix-shell instead of ${pkgs.runtimeShell}