Probably [[https://www.mediawiki.org/wiki/Manual:$wgResourceBasePath|$wgResourceBasePath]], which we can then customize in our GlobalSettings.php, like how we customized $wgStylePath and $wgExtensionAssetsPath to improve Varnish hit rates.
Right now the current code results in entries like this being written to the generated <wiki subdomain>Settings.php files:

$wgRightsIcon = "$wgScriptPath/resources/assets/licenses/cc-by-sa.png";

But as the MW.org manual page notes, $wgScriptPath is a relative path, hence not what we want to use here.