Update rc to use offline gem cache like nightly

I've done some testing:

Nightly installs cleanly on debian and ubuntu, so the principle of the new format is good
Rc1 (old format) -> Nightly upgrades fine on ubuntu 32bit (not tested the others yet) so upgrading to the new format should be ok.

More testing is ongoing, but I want to get a second pair of eyes on this while I do it