fix: ensure overlays are passed to all possible nixpkgs instances

[srounce]

Tried to apply an overlay to get around the current boost 1.89 situation on nixpkgs only to find it wasn't actually being applied everywhere and neither was nixpkgs.config. Added an extra module to ensure that they are passed to config.nixpkgs too.

[devurandom]

This would fix #115, where it is reported that the config is applied to Home Manager, but not to NixOS.

I was interested to compared the approach of this PR with how it works for Home Manager:

blueprint/lib/default.nix

Lines 231 to 237 in 06ee719

	mkHomeUsersModule =
	hostname: homeManagerModule:
	let
	module =
	{ perSystem, config, ... }:
	{
	imports = [ homeManagerModule ];

blueprint/lib/default.nix

Lines 343 to 361 in 06ee719

	hosts = importDir (src + "/hosts") (
	entries:
	let
	loadDefaultFn = { class, value }@inputs: inputs;
	loadDefault = hostName: path: loadDefaultFn (import path { inherit flake inputs hostName; });
	loadNixOS = hostName: path: {
	class = "nixos";
	value = inputs.nixpkgs.lib.nixosSystem {
	modules = [
	perSystemModule
	path
	] ++ mkHomeUsersModule hostName home-manager.nixosModules.default;
	specialArgs = specialArgs // {
	inherit hostName;
	};
	};
	};

home-manager.nixosModules.default is:

https://github.com/nix-community/home-manager/blob/7ba4ee4228ed36123c7cb75d50524b43514ef992/flake.nix#L13-L16

https://github.com/nix-community/home-manager/blob/7ba4ee4228ed36123c7cb75d50524b43514ef992/nixos/default.nix#L47

https://github.com/nix-community/home-manager/blob/7ba4ee4228ed36123c7cb75d50524b43514ef992/nixos/common.nix#L40-L47

https://github.com/nix-community/home-manager/blob/7ba4ee4228ed36123c7cb75d50524b43514ef992/modules/modules.nix#L82

https://github.com/nix-community/home-manager/blob/7ba4ee4228ed36123c7cb75d50524b43514ef992/modules/misc/nixpkgs-disabled.nix

I was unable to figure out how nixpkgs.config ends up being used by home-manager.nixosModules.default (under blueprint's setting of useGlobalPkgs = true, which sets useNixpkgsModule = false), neither in the code traced here nor in the surrounding code.

Could you please explain why this happens in the first place for NixOS / why it does not happen within home manager?

[r-vdp]

Could you please explain why this happens in the first place for NixOS / why it does not happen within home manager?

You traced the HM-inside-NixOS path (home-manager.nixosModules.default → useGlobalPkgs = true → nixpkgs-disabled.nix).
On that path the config is indeed never applied — HM just inherits the NixOS host's pkgs, and that pkgs is the unconfigured one (see below).

The path where it does work is the standalone HM entry point, which is what the #115 reporter inspected (legacyPackages.x86_64-linux.homeConfigurations."me@my-nixos"):

blueprint/lib/default.nix

Lines 330 to 341 in 06ee719

	eachSystem (
	{ pkgs, system, ... }:
	{
	homeConfigurations = lib.mapAttrs (
	_name: homeData:
	mkHomeConfiguration {
	inherit (homeData) modulePath username;
	inherit pkgs system;
	}
	) homesFlat;
	}
	);

blueprint/lib/default.nix

Lines 293 to 294 in 06ee719

	home-manager.lib.homeManagerConfiguration {
	inherit pkgs;

Here pkgs = systemArgs.${system}.pkgs (the instance blueprint built with the configured nixpkgs.config/overlays) is handed straight to homeManagerConfiguration.
HM's modules/misc/nixpkgs.nix then mirrors pkgs.config into config.nixpkgs.config, which is why the value shows up there.

loadNixOS on the other hand, calls nixosSystem with no pkgs at all:

blueprint/lib/default.nix

Lines 350 to 360 in 06ee719

	loadNixOS = hostName: path: {
	class = "nixos";
	value = inputs.nixpkgs.lib.nixosSystem {
	modules = [
	perSystemModule
	path
	] ++ mkHomeUsersModule hostName home-manager.nixosModules.default;
	specialArgs = specialArgs // {
	inherit hostName;
	};
	};

So the NixOS module system instantiates its own from config.nixpkgs.hostPlatform, ignoring blueprint's settings — and HM-inside-NixOS inherits that via useGlobalPkgs.

I've pushed a follow-up commit that fixes it the same way standalone HM already works: hand over systemArgs.${system}.pkgs via nixpkgs.pkgs (rather than setting nixpkgs.config/overlays, which would make NixOS import nixpkgs a second time per host with the same settings).

Also rebased onto current main so CI passes (#151 fixed an unrelated template breakage).