cpython: allow $out reference again

This regression got introduced with 738746a34b

pkgs/development/interpreters/python/cpython/default.nix

@@ -330,6 +330,7 @@ in with passthru; stdenv.mkDerivation {
   postInstall = let
     # References *not* to nuke from (sys)config files
     keep-references = concatMapStringsSep " " (val: "-e ${val}") ([
+      (placeholder "out")
     ] ++ optionals tzdataSupport [
       tzdata
     ]);