fix(home): move nix.package fallback to darwin module

home/base/home.nix

@@ -1,4 +1,4 @@
-{ myvars, pkgs, ... }:
+{ myvars, ... }:
 {
   # Home Manager needs a bit of information about you and the
   # paths it should manage.
@@ -15,6 +15,4 @@
     # changes in each release.
     stateVersion = "24.11";
   };
-
-  nix.package = pkgs.nix;
 }

home/darwin/default.nix

@@ -1,10 +1,16 @@
 {
+  lib,
   mylib,
   myvars,
+  pkgs,
   ...
 }:
 {
   home.homeDirectory = "/Users/${myvars.username}";
+
+  # home-manager's darwin module only forwards the system-level nix.package
+  # when `nix.enable` is on, so provide a low-priority fallback here.
+  nix.package = lib.mkDefault pkgs.nix;
   imports = (mylib.scanPaths ./.) ++ [
     ../base/core
     ../base/tui