nix-darwin,nixos: convert modulesPath to string (#2714)

`modulesPath` is usually used with antiquotation
(`"${modulesPath}/some-module.nix"`). Since antiquoted paths are copied
to the Nix store, one must explicitly do `"${toString
modulesPath}/some-module.nix"` to avoid that. Ideally `modulesPath`
should be a string to avoid this. Note that `modulesPath` is already
defined as a string in <home-manager>/modules/default.nix and
<nixpkgs>/nixos/lib/eval-config.nix.

nix-darwin/default.nix

@@ -13,7 +13,7 @@ let
       lib = extendedLib;
       darwinConfig = config;
       osConfig = config;
-      modulesPath = ../modules;
+      modulesPath = builtins.toString ../modules;
     } // cfg.extraSpecialArgs;
     modules = [
       ({ name, ... }: {