support multiple instances of nixpkgs

[zimbatm]

At the moment, only the "nixpkgs" input is used to:

• instantiate nixos instances.
• configure the pkgs argument.

Some users might want to use different versions of nixpkgs for different machines (eg: stable and unstable)

[kriswill]

or, use multiple nixpkgs? For instance, I like my OS nixpkgs to be pinned to the current nixos-release tag, but use certain applications from the unstable, or even trunk...

[zimbatm]

The current workaround is to create a modules/common/nixpkgs-unstable.nix module.

{ inputs, pkgs, .... }:
{
  _module.args.pkgsUnstable = import inputs.nixpkgs-unstable {
    inherit (pkgs) system;
    config.allowUnfree = true;
  };
}

Then import it in whatever module needs it:

{ flake, pkgsUnstable, ... }:
{
  # Import to add the pkgsUnstable argument
  imports = [ flake.modules.common.nixpkgs-unstable ];
  # Pull the package from pkgsUnstable
  services.myservice.package = pkgsUnstable.myservice;
}