pk: Set cache-age for distro-upgrade transaction

The behaviour of different backends with respect to cache expiry is not unique with the default cache-age setting.

In Fedora by default the cache is never expired. Instead, a patch downstream is implemented to force refresh the cache. However, that leaves untouched the cache of the non-current distro releases, as used by UpgradeSystem. It is thus important that we explicitly specify a cache-age hint to unequivocally expire the cache after a while.