scoru: fix the proof of the rollup node

The rollup-node currently use the latest version of its inbox. Whereas the snapshot of the refutation game can be much older. It is then able to lose a refutation game because it uses the wrong snapshot. @vch9

see !6118 (comment 1080808770), !6118 (comment 1082182937), !6118 (comment 1082188898) and !6118 (comment 1082193561)