Supply the --nix flag to stack if we appear to be on Nix/NixOS

[hdgarrood]

Raised as a possibility in #21 . Currently, building purs on Nix is unlikely to work, as stack setup reports:

I don't know how to install GHC on your system configuration, please install manually

I honestly have no idea how good of an idea this is; if we can't do this reliably then I think it could easily cause more pain than it would solve. Perhaps we should just not support Nix at all; Nix users do have other options for installing the compiler.

[ae-mo]

Any updates? I'm having the same issue described in #21

[hdgarrood]

I'm still not convinced this is a good idea at all, but I don't understand Nix well enough to say either way (and realistically I'm not going to learn enough about Nix to be able to decide any time soon). I recommend https://github.com/justinwoo/easy-purescript-nix instead.