[feat] Propagate sys.path as PYTHONPATH while develop

While invoke setup.py certain eggs (like scikit-learn) launch
cetain custom builds (for cython) using subprocess and sys.executable.

This commit aims to preserve the sys.path over the runs, even if an
egg is using subprocess with the same python to build a component of
the egg.

src/zc/buildout/easy_install.py

@@ -1619,6 +1619,8 @@ for extra_path in %%(path_list)r:
 
 import os, setuptools
 
+os.environ['PYTHONPATH'] = (os.pathsep).join(sys.path[:])
+
 __file__ = %%(__file__)r
 
 os.chdir(%%(setupdir)r)