Flake support

[utdemir]

The upcoming flakes feature of Nix simplifies the setup on that project, so I think we need to migrate to it once it is released.

Actually, using https://github.com/edolstra/flake-compat we could even use the flake-based setup today, however it lacks many niceties coming with flakes, so I still think we should wait for an official release.

An example Haskell project using flakes is here: https://github.com/utdemir/nix-tree

[frontsideair]

This flake.nix is working perfectly for my use case. Thanks for the help @utdemir!