Haskell.nix Nixpkgs Pin

Haskell.nix contains several Nixpkgs pins imanaged by niv in nix/sources.json.

These are used in testing various versions of nixpkgs.

To use haskell.nix the config and overlays need to be applied to Nixpkgs. Users should probably pin a suitable version of nixpkgs, although things might not work for them if their Nixpkgs version is too different.

We aim to keep this pin somewhere on a channel of the Nixpkgs latest stable release. That is currently 20.09.

We also execute tests on MacOS (darwin). The darwin channel is usually behind the NixOS channel. So we follow the nixpkgs-20.09-darwin channel.