Consider including .mix directory into buildMix output

Reported by @goodsoft. It seems that unless .mix is persisted, Dialyzer cannot be used during check phase. Which would mean it has to be preserved in the output.

One way to do it could be:

{ pkgs ? import ./pkgs.nix {} }: with pkgs;

let
  inherit (callPackage ./. {}) mixToNix;

  exposeDotMix = drv: drv.overrideAttrs (super: {
    postInstall = ''
      mv .mix $out || true
    '';
  });
in

mixToNix {
  src = ./tests/01-poison;
  overlay = final: lib.mapAttrs (lib.const exposeDotMix);
}