If self.config.build_url is set, ensure it's been downloaded to the
        builds folder.

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()