SCORU/Node: fix incorrect inbox for migration blocks

Context

This MR fixes an issue with the way the migration block is interpreted inside the rollup node. There was an off-by-one error in the detection of the first block of a protocol which made the rollup node disagree with the inbox on L1 (the rollup node would then stop and enter degraded mode).

The actual fix is contained in the first commit of this MR.

Manually testing the MR

dune exec tezt/tests/main.exe -- -f sc_rollup.ml store  migration