Support MIX_ARCHIVES

[Devin Alexander Torres]

We need exenv to change MIX_ARCHIVES env var to point to a version specific archives folder.

[Akash Manohar]

👍 Need this.

Currently have problems when using Mix (hex to be specific) with really old Elixir versions like 0.13.0 and new version 0.15.0.

[Kenta Nakase]

👍 I was struggling on the weird error on older phoenix + elixir with newer hex.

[Yuki Ito]

Ok, we need this definitely. But I don't have enough time to tackle this during the next two weeks. Pull requests are welcome!