Getting the logs wet and mold growing on them unfortunately is something we all have to deal with. Ideally we would cut the logs and build the walls as soon as possible and get under roof before they get wet (unlikely but possible). Covering the logs while they are stacked is of course preferable but not always an option. I would try and get your logs up as quickly as possible and then borax them when you get the roof on. It’s easier to do them while they are on the ground but if they keep getting wet you will be constantly scrubbing logs. When the walls are built it may be harder to get to the logs and take more time but at least once done its done.