code: def run(self):
          
          env_name = self.env_name or BUILD_JSON_ENV
          try:
              build_cfg_json = os.environ[env_name]
          except KeyError:
              self.log.error("build config not found in env variable '%s'", env_name)
              return None
          else:
              try:
                  return self.substitute_configuration(json.loads(build_cfg_json))
              except ValueError:
                  self.log.error("couldn't load build config: invalid json")
                  return None
docs: get json with build config from environment variable