scoru: fix production proof of rollup node

Context

The refutation proof produced by the rollup node use the inbox of the current level. This MR changes that to use the snapshotted inbox (inbox at the level of the creation of the game).

fix #3744 (closed)

Manually testing the MR

run two nodes (looser vs honest).

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR