Allow MIX_ENV/MIX_TARGET settings to pass through

[fhunleth]

Prior to this change, MIX_ENV=test was forced and any preferred_cli_env and preferred_cli_target configuration in the mix.exs would look like it was ignored. In actuality, preferred_cli_* wasn't ignored, but you'd have to set it on do based on how ANSI color was enabled.

This change enables ANSI a different way so that the mix target is passed through and can get the preferred_cli_* settings, if any. Because of this MIX_ENV doesn't need to be forced any more.