For anyone who is looking for this later: this has been fixed in Elixir 1.9 or later with the introduction of MIX_BUILD_PATH. There is a pending fix in Elixir 1.10 that ensures that the MIX_BUILD_PATH still has the environment appended. See PR#9189.
NOTE: I found my post here while looking for something else, because I want to be able to add the Elixir and/or Erlang version to the build path, and the apparent recommendation is to set the env_path, not the build_path. This will make it possible for me to build the same application under multiple versions of Elixir so that I can test newer versions before I want to upgrade or during development. I have to test this, but I think that I will be setting env_path to be something like Pathname.join(["_build", System.version()]).
