docs: def ensure_build(self):
          """
          If self.config.build_url is set, ensure it's been downloaded to the
          builds folder.
          """
          if self.config.build_url:
              path = get_buildfile_path(self.config)
  
              # Ensure that builds_root has been created.
              mkdir(BUILDS_ROOT)
              build_md5 = getattr(self.config, 'build_md5', None)
              ensure_file(self.config.build_url, path, build_md5)
  
              # Now untar.
              self.untar()