[BFSS] mark fs_imported value as uncachable
This will finally fix the `test_bfss_fs_importing_transparency` pass again.
Regression was introduced by 4ba11607d84a
# -*- coding: utf-8 -*-
# copyright %(year)s %(author)s, all rights reserved.
# contact %(author-web-site)s -- mailto:%(author-email)s
#
%(long-license)s
"""%(distname)s specific hooks and operations"""