helpsite url can't be localised at install time

Localised distributions can use the localise.xml file to force a default helpsite for new installs. This has been broken since 1.6.

To test: patch and add this url in installation/localise.xml, then install a clean Joomla


	<helpurl>http://help.joomla.fr/3/index.php?option=com_help&amp;keyref=Help{major}{minor}:{keyref}</helpurl>
	
If none set the joomla.org en-GB one will be used