sui_v1.78.0_1786648392_ci: fix the source-verification-matrix nightly test (#27657)

Problem The nightly source-verification version matrix has been failing, and the cause is the chain identifier the test writes into its fixture. Ask for --format hex explicitly, and assert the result is eight hex digits at the point it is read. chain_identifier now returns String rather than Option<String> , since there is no longer a case where a missing value is meaningful — a CLI that cannot…