Proto: Scoru: fix encoding when reading reveal_hash

Context

part of #4201 (closed)

Found during development of an e2e kernel tezt test, which uses the reveal hash mechanism. The incorrect decoding was causing the rollup node to ask for the preimage of the 'zero hash', which broke the tezt test.

Manually testing the MR

A tezt test MR testing the entire flow, including DAC, will be opened next week.

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