add nixproxy.nix to grab pkgs from repl's replit.nix

[turbio]

give us a bit of future leeway on how we structure the nix experience. This expects a replit.nix in the repldir giving us the set deps which are passed right into nix-shell.

for example, something like:

{ pkgs }: {
    deps = [
                pkgs.python39
    ];
}