[BFSS] mark fs_imported value as uncachable
This will finally fix the `test_bfss_fs_importing_transparency` pass again.
Regression was introduced by 4ba11607d84a
#! /bin/sh -e#if [ "$1" = configure ]; then if which install-docs >/dev/null 2>&1; then install-docs -i /usr/share/doc-base/cubicweb-doc fifi#DEBHELPER#exit 0