Add populate-cache-effect

[zmrocze]

Motivation

We want to push some paths to either the cachix or attic cache, additionaly to the cache configured for herculesCI. We discussed it recently on mlabs slack. If you guys would be interested here's a module doing that to be used like below:

populate-cache-effect = {
      enable = true;
      caches = {
        mlabs-attic = {
          type = "attic";
          secretName = "cardano-nix-attic-token";
          branches = ["master"];
          packages = [test-drv];
        };
        mlabs-cachix = {
          type = "cachix";
          secretName = "karol-cachix-token";
          branches = ["master" "push-to-attic"];
          packages = [test-drv];
        };
      };
    };

it's a flake-parts module producing effects under herculesCI.onPush.default.outputs.effects.populate-cache-effect option.

Questions

It works. But I don't understand the structure of this repository, so I'd like to ask first how to incorporate that into the codebase:

• where/how to export this
• if it's fine the way it is with default attic-client-pkg cachix-pkg

Maintainer checklist

• Documentation (function reference docs, setup guide, option reference docs)
• Has tests (VM test, free account, and/or test instructions)
• Is the corresponding module up to date?

[zmrocze]

I don't have access to this repository, so can't push to a branch at mlabs-haskell/hercules-ci-effects.

[brainrake]

Closing in favor of hercules-ci#165