merge PYTHONEXTRAPATH value into PYTHONPATH in easy_install.

src/zc/buildout/easy_install.py

@@ -704,6 +704,9 @@ class Installer:
         tmp = tempfile.mkdtemp(dir=dest)
         try:
             path = setuptools_loc
+            extra_path = os.environ.get('PYTHONEXTRAPATH')
+            if extra_path:
+                path = os.pathsep.join([path, extra_path])
 
             args = ('-c', self._easy_install_cmd, '-mUNxd', _safe_arg(tmp))
             if not self._has_broken_dash_S: