gnome-extra/gnome-software: bump to 3.30.6, conditional packagekit
packagekit is "not ready for primetime" and even use.masked, so I don't
think we should force it on gnome-software when gnome-software can be
useful for upgrading device firmware, handle shell extensions and
(eventually) flatpaks.
Also default-enable USE=firmware to have firmware upgrades show up,
some of which may be security relevant (e.g. logitech unifying receiver
firmware updates).
Package-Manager: Portage-2.3.52, Repoman-2.3.12
Signed-off-by: Mart Raudsepp <leio@gentoo.org>