add nixpkgs input to nixpath

this is required because with Flakes we don't want to use channels.

flake.nix

@@ -30,6 +30,7 @@
           config.allowUnfree = true;
         };
       };
+
       pkgs = import nixpkgs {
         inherit system;
         config = {
@@ -41,7 +42,7 @@
       };
       mkComputer = configurationNix: homeManagerRole: extraModules: nixpkgs.lib.nixosSystem {
         inherit system pkgs;
-        specialArgs = { inherit self system inputs username; };
+        specialArgs = { inherit self nixpkgs system inputs username; };
         modules = (
           [
             # System configuration for this host

modules/common/default.nix

@@ -1,4 +1,4 @@
-{ self, pkgs, username, ... }:
+{ self, nixpkgs, pkgs, username, ... }:
 {
   imports = [
     "${self}/modules/cli"
@@ -67,6 +67,15 @@
   nixpkgs.config.allowUnfree = true;
 
   nix = {
+    nixPath = [ "nixpkgs=${nixpkgs}" ];
+    registry.nixpkgs = {
+      from = {
+        id = "nixpkgs";
+        type = "indirect";
+      };
+      flake = nixpkgs;
+    };
+
     autoOptimiseStore = true;
     package = pkgs.nixFlakes;
     extraOptions = ''