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.