Analize Nix flakes support

[cript0nauta]

I suppose it would replace pynixify/nixpkgs.nix for a flake.nix. Nixpkgs pinning would be done with flakes instead of with the --nixpkgs option. Also, local development could benefit of dirty Git tree detection.

Useful link: https://www.tweag.io/blog/2020-05-25-flakes/

[mikepurvis]

Would love to have native flake support, definitely!