nixos/documentation.nix: Only use store non-flake pkgs.path directly when already copied

nixos/modules/misc/documentation.nix

@@ -71,8 +71,14 @@ let
         #   produce separate, smaller store paths
         # When already in the store,
         #   avoid copying; reuse the whole nixpkgs sources
+        #
+        # We can only avoid copying when pkgs.path is already a string. A path
+        # value can not be converted to a store path without rehashing it.
+        # builtins.storePath would be a solution but is currently off-limits
+        # because of https://github.com/NixOS/nix/issues/1888
+        #        and https://github.com/NixOS/nix/issues/5868
         pull = dir:
-          if isStorePath pkgs.path
+          if builtins.typeOf pkgs.path == "string" && isStorePath pkgs.path
           then "${pkgs.path}/${dir}"
           else filter "${toString pkgs.path}/${dir}";
       in