Cannot use Silk icon theme anymore

1. Go to the administration > presentation.
2. Select "Silk" for the icon theme, and save
3. Go back to the home, you see in the App Bar that the icons are not displayed

Introduced by XWIKI-11400.

The Silk rendering use $xwiki.getSkinFile() but now the velocity engine runs in its own context and namespace, so $xwiki does not exists there.