docs: def env(self, **kw):
          '''
          Allows adding/overriding env vars in the execution context.
          :param kw: Key-value pairs
          :return: self
          '''
          self._original_env = kw
          if self._env is None:
              self._env = dict(os.environ)
          self._env.update({k: unicode(v) for k, v in kw.iteritems()})
          return self