nix_health: Flake-specific health checks

[srid]

Each flake can export the metadata (as https://github.com/DeterminateSystems/flake-schemas?) for nix_health do checks on. This includes:

• Which Nix caches to check
• Does the project require/recommend direnv? If so, check for direnv allow state Add direnv health check (flake-specific) #70
• Minimum recommended memory

{
  flake.nix-health.default.checks = {
    caches.required = [ "nammyatri.cachix.org" ];
    direnv.enable = true;
  };
}

Some of these checks (like direnv) will require a specific flake to check against. In the case of direnv, it would also have to be a local path flake.

[shivaraj-bh]

Regarding direnv health check, this is how I did it in nushell: https://github.com/shivaraj-bh/nix-health/blob/8de00ad118a14f3d3d1b75f296697cb7f9012a26/nix-health.nu#L77-L97

[srid]

With #67 we can now configure nix-health behaviour in the project flake as follows:

[srid]

This is architecturally in place. #14 will track the specific checks.