`stack clean --full` may not work properly with docker + nix

[mgsloan]

To fix #2010, "stack clean" was made to no longer run within a docker or nix environment. This broke cleaning within the nix-shell as described in #3468. The solution in #3470 is to not enter docker for stack clean --full, but enter into nix. I don't think this will work properly for docker + nix.

A better solution might be to make stack clean --full work within the containers, by not trying to delete the _home dir mentioned in #2010

[snoyberg]

I've reworked stack clean to no longer use an EnvConfig at all, which should fully address these concerns. Closing.