Announcing Mixnix - Build Elixir projects with Nix

It fails immediately with

17:49:44.708 [error] No configuration file found
17:49:44.708 [error] No configuration file found
** (Mix) The task "compile.phoenix" could not be found. Did you mean "compile.elixir"?
** (Mix) The task "compile.phoenix" could not be found. Did you mean "compile.elixir"?

Unsure of what I’m doing wrong:

{ buildMix, fetchgit }:
let
  src = fetchgit {
    url = "https://framagit.org/framasoft/mobilizon";
    rev = "7c43d52b20a61a4db30536e4d3bfda398a8c6603";
    sha256 = "0v8rwd9i967mpi69adii8zjmh7v4srkjsfjxrnnphszp9k6yvl1d";
  };
in
  buildMix {
    name = "mobilizon";
    version = "1.0.0";
    inherit src;
  }

is my expression.