in process of building project mix have req of placing result of build in dir.
mix places build artefacts in /project_path/_build.
cannot write /project_path during actual build.
can change output dir? can adjusted?
based on this, can specify keyword list member build_path override default _build directory.
example:
def project [app: :my_app, version: "0.0.1", elixir: "~> 1.2", build_embedded: mix.env == :prod, start_permanent: mix.env == :prod, build_path: "custom_build_dir", deps: deps] end 