core: fix download menu

Currently, when one enables the "Only allow downloads from primary"
option, all the other mirror options are hidden (expected!) but the
LuaRocks and CPAN mirrors (bad!).

Fix that.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>