package/kodi: share the download dir with helper programs

The two helper programs TesturePackager and JsonSchemaBuilder are built
out of the same source tree as Kodi, so to avoid downloading the same
archive three times, let's them share the same download directory.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>