Provide up-to-date Cachix of nix-tools

[rvl]

Users might prefer to use Cachix instead of the IOHK Hydra cache.

• Set up the Haskell.nix Buildkite pipeline to save a cache of the nix-tools build.
• Add documentation on how to use the cached build.

This probably requires pinning nixpkgs (#98).

[angerman]

nix-tools.cachix.org is now populated by hercules-ci, which runs on haskell.nix for linux and darwin.

[amarrella]

Hi @angerman, thanks for the update! :)
I'm trying to build the tools leveraging the cache but it doesn't seem to use it. I tried both on current master and on the latest successful hercules build (i see build failures in the latest commits).

I'm trying to build on darwin. Any tip on how to speed up the build? My computer has been trying to build

nix build '(with import <nixpkgs> (import (builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz)); haskell-nix.nix-tools)' --out-link nt

but it's taking hours :(

[wedens]

@amarrella It's currently cached fot nixpkgs defined here.

So, if you replace your <nixpkgs> with that, it'll use cached nix-tools.

[amarrella]

@wedens: that works, thank you :)

[chrissound]

In case it's helpful to someone else:

Here is a nix expression to build the nix-tools utilizing the cachix cache :

{
  nixpkgs ? (import (builtins.fetchTarball "https://github.com/input-output-hk/nixpkgs/archive/a8f81dc037a5977414a356dd068f2621b3c89b60.tar.gz") ( haskellnix ))
, haskellnix ? (import (builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz))
} :
(nixpkgs.haskell-nix.nix-tools)

[masaeedu]

@angerman Is it still necessary to use input-output-hk/nixpkgs instead of the standard nixpkgs to get cache hits? If so, it would be nice to mention in the README shortly after this bit (or as a qualification to it):

that if the user wants to avoid building everything locally there is still a dependency on iohk-nixpkgs.