refactor(buildClan): simplify pkgs overriding logic

lib/build-clan/machineModules/forSystem.nix

@@ -1,26 +0,0 @@
-{
-  pkgs,
-  pkgsForSystem,
-  system,
-}:
-{
-  lib,
-  ...
-}:
-{
-  imports = [
-    (
-      {
-        # For vars we need to override the system so we run vars
-        # generators on the machine that runs `clan vars generate`. If a
-        # users is using the `pkgsForSystem`, we don't set
-        # nixpkgs.hostPlatform it would conflict with the `nixpkgs.pkgs`
-        # option.
-        nixpkgs.hostPlatform = lib.mkIf (system != null && (pkgsForSystem system) != null) (
-          lib.mkForce system
-        );
-      }
-      // lib.optionalAttrs (pkgs != null) { nixpkgs.pkgs = lib.mkForce pkgs; }
-    )
-  ];
-}

lib/build-clan/machineModules/overridePkgs.nix

@@ -0,0 +1,17 @@
+{
+  pkgs,
+}:
+{
+  lib,
+  ...
+}:
+{
+  imports = [
+    ({
+      # For vars we need to ensure that the system so we run vars generate on
+      # is in sync with the pkgs of the system
+      nixpkgs.hostPlatform = lib.mkForce pkgs.system;
+      nixpkgs.pkgs = lib.mkForce pkgs;
+    })
+  ];
+}