Reset environment variables on rebootstrap

This is useful when a recipe modifies the environment during install/update.

slapos/rebootstrap/__init__.py

@@ -19,6 +19,7 @@ from zc.buildout import easy_install, UserError
 class extension(object):
 
   def __init__(self, buildout):
+    self.environ = os.environ.copy()
     self.buildout = buildout
     # fetch section to build python (default value is 'buildout')
     self.python_section = buildout['buildout']['python'].strip()
@@ -81,7 +82,7 @@ Buildout will be restarted automatically to have this change applied.
             ] + list(map(cache, get_distributions())))
 
     shutil.copy(new_bin, installed)
-    os.execv(self.wanted_python, [self.wanted_python] + sys.argv)
+    os.execve(self.wanted_python, [self.wanted_python] + sys.argv, self.environ)
 
   def _compute_part_signatures(self, install_parts):
     # Skip signature check for parts that were required to build the wanted