component/firefox: set $GSETTINGS_SCHEMA_DIR in the wrapper

This fixes a crash happening after `setFile` selenium command under
some conditions (it does not always happen, but I did not investigate
in which conditions it happens exactly).

[Parent 94283, Main Thread] ###!!! ASSERTION: No GSettings schemas are installed on the system: 'glib assertion', file /builds/worker/checkouts/gecko/toolkit/xre/nsSigHandlers.cpp:164