build(python): align toolchain with monorepo pattern

configs/nixos/modules/shell/python.nix

@@ -13,6 +13,8 @@ in
   options.modules.shell.python.enable = lib.mkEnableOption "Python tools for the primary user's shell environment";
 
   config = lib.mkIf cfg.enable {
+    registry.env-var.${config.networking.hostName}.PYTHON_TOOLCHAIN_ROOT = "${pkgs.python3}";
+
     home-manager.users.${user}.home.packages = [ pkgs.python3 ];
   };
 }