pathDerivation: Copy path

Otherwise you end up with a derivation that refers to the original
path (which is not in the Nix store and not accessible to
builders). This caused the derivation paths for the docbookrx package
(removed on master) to depend on the location of the nixpkgs source
tree.

pkgs/development/ruby-modules/bundled-common/functions.nix

@@ -76,7 +76,7 @@ in rec {
           bundledByPath = true;
           name = gemName;
           version = version;
-          outPath = path;
+          outPath = "${path}";
           outputs = [ "out" ];
           out = res;
           outputName = "out";