docs: Use a nixpkgs patch instead of copy/pasting nixpkgs functions (#1011)

This makes the code more maintainable, as it only depends on our small
patch, instead of a large number of internals of nixpkgs.

flake-modules/modules.nix

@@ -31,13 +31,12 @@
 
   perSystem = {
     pkgs,
-    pkgsUnfree,
     config,
     ...
   }: {
     _module.args = {
       modules = modules pkgs;
-      modulesUnfree = modules pkgsUnfree;
+      rawModules = modules;
     };
   };
 }