Check nightly build folders the same way we check inbound

Hmm, I agree that it may be unclear. I don't really like the idea to make an accessor to _cache since it contains some really cached information - it is used as a list with 2 elements, the first being the state of the downloaded (or not) data, and the second is user data associated to this index. We could find another way to do this - however here it is just a move of the code (that was previously here) so we could handle this in a following step I suppose.