versions_have_idempotent_up test fails with "Failed to ROLLBACK failed transaction...there is no transaction in progress"

Test omicron-nexus::test_all integration_tests::schema::versions_have_idempotent_up failed with

thread 'integration_tests::schema::versions_have_idempotent_up' panicked at 'Failed to ROLLBACK failed transaction: Error { kind: Db, cause: Some(DbError { severity: "ERROR", parsed_severity: Some(Error), code: SqlState(Other("XXUUU")), message: "there is no transaction in progress", detail: None, hint: None, position: None, where_: None, schema: None, table: None, column: None, datatype: None, constraint: None, file: Some("exec_util.go"), line: Some(741), routine: Some("init") }) }', nexus/tests/integration_tests/schema.rs:96:22

Buildomat log

From @davepacheco:

• looks like we executed BEGIN; and the update SQL, then COMMIT;
• the first two succeeded (or we would have panicked)
• the COMMIT; failed and CockroachDB reports restart transaction: TransactionRetryWithProtoRefreshError: TransactionRetryError: retry txn (RETRY_SERIALIZABLE)
• apply_update_as_transaction then issues a ROLLBACK; but that fails because the transaction is no longer in progress
• so we panic

It's not clear to me why we'd ever expect to get a serializeable error here or what the logic here is trying to do in terms of retrying (or why it's issuing a rollback, since the only that can have failed here is the commit