nixos/lib/eval-config.nix: toString modulesPath

Referencing modulesPath in NixOS configurations can cause evaluation
errors in restricted mode.  If used as `${modulesPath}` (as in all
use-sites in nixpkgs) the modules subtree is copied into its own store
path. Access to this path will be forbidden in restricted mode.

Converting to a string solves this issue.
`${builtins.toString modulesPath}` will point to a subdirectory of the
nixpkgs tree out of which evalModules is called.

This change converts modulesPath to a string by default so that the
call-site doesn't have to anymore.

nixos/lib/eval-config.nix

@@ -49,7 +49,8 @@ in rec {
     inherit prefix check;
     modules = modules ++ extraModules ++ baseModules ++ [ pkgsModule ];
     args = extraArgs;
-    specialArgs = { modulesPath = ../modules; } // specialArgs;
+    specialArgs =
+      { modulesPath = builtins.toString ../modules; } // specialArgs;
   }) config options;
 
   # These are the extra arguments passed to every module.  In