python.pkgs.twisted: move twisted cache generation to postFixup

The Python 2 build of twisted would fail when generating the cache. This
failure started occuring since the change was introduced that builds
setuptools as a wheel instead of as an egg,
90be4c2c78.

Turns out moving this step to postFixup solves the issue. I suppose it
is related to the order of site packages being evaluated, where
PYTHONPATH introduced additional ones.

pkgs/development/python-modules/twisted/default.nix

@@ -39,7 +39,9 @@ buildPythonPackage rec {
   # http://twistedmatrix.com/documents/current/core/howto/plugin.html#auto3
   # and http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=477103 for
   # details.
-  postInstall = "$out/bin/twistd --help > /dev/null";
+  postFixup = ''
+    $out/bin/twistd --help > /dev/null
+  '';
 
   checkPhase = ''
     ${python.interpreter} -m unittest discover -s twisted/test