treewide: reduce number of evaluations by re-using pkgs

This means:
- For home-manager standalone, we will re-use the
  `self.outputs.legacyPackages`
- For NixOS, we will still eval its own pkgs, but we will pass
  `useGlobalPkgs` option to Home-Manager

home-manager/default.nix

@@ -1,4 +1,4 @@
-{ pkgs, lib, flake, osConfig, ... }:
+{ pkgs, lib, osConfig, ... }:
 
 {
   imports = [
@@ -15,8 +15,6 @@
   # Inherit config from NixOS or homeConfigurations
   inherit (osConfig) device mainUser;
 
-  nixpkgs.overlays = [ (import ../overlays { inherit flake; }) ];
-
   # Assume that this is a non-NixOS system
   targets.genericLinux.enable = lib.mkIf pkgs.stdenv.isLinux (lib.mkDefault true);
 }

lib/flake-helpers.nix

@@ -7,7 +7,7 @@ in
   mkGHActionsYAMLs = names: eachDefaultSystem (system:
     let
       inherit (nixpkgs) lib;
-      pkgs = nixpkgs.legacyPackages.${system};
+      pkgs = self.outputs.legacyPackages.${system};
       mkGHActionsYAML = name:
         let
           file = import (../actions/${name}.nix);
@@ -38,7 +38,7 @@ in
     , deps ? pkgs: [ ]
     }: eachDefaultSystem (system:
     let
-      pkgs = nixpkgs.legacyPackages.${system};
+      pkgs = self.outputs.legacyPackages.${system};
     in
     {
       apps.${name} = mkApp {
@@ -102,7 +102,7 @@ in
     }:
     {
       homeConfigurations.${hostname} = home-manager.lib.homeManagerConfiguration {
-        pkgs = nixpkgs.legacyPackages.${system};
+        pkgs = self.outputs.legacyPackages.${system};
         modules = [
           ({ ... }: {
             home = { inherit username homeDirectory; };

nixos/home.nix

@@ -22,6 +22,7 @@ in
 
     home-manager = {
       useUserPackages = true;
+      useGlobalPkgs = true;
       users.${config.mainUser.username} = {
         inherit (config.nixos.home) imports;
       };