[BFSS] mark fs_imported value as uncachable
This will finally fix the `test_bfss_fs_importing_transparency` pass again.
Regression was introduced by 4ba11607d84a
<html>
<head>
<script type="text/javascript" src="../../data/jquery.js"></script>
<script src="../../data/jquery.corner.js" type="text/javascript"></script>
<script src="../../data/cubicweb.js" type="text/javascript"></script>
<script src="../../data/cubicweb.python.js" type="text/javascript"></script>
<script src="../../data/cubicweb.compat.js" type="text/javascript"></script>
<script src="utils.js" type="text/javascript"></script>
<script type="text/javascript" src="qunit.js"></script>
<link rel="stylesheet" type="text/css" media="all" href="qunit.css" />
<script src="cwmock.js" type="text/javascript"></script>
<script src="test_utils.js" type="text/javascript"></script>
</head>
<body>
<div id="main">
</div>
<h1 id="qunit-header">cw.utils functions tests</h1>
<h2 id="qunit-banner"></h2>
<h2 id="qunit-userAgent"></h2>
<ol id="qunit-tests">
</body>
</html>