follow-up after fixing Nondeterministic DDL

[lance6716]

What did you do?

1. modification to job.ddls should be as late as possible https://github.com/pingcap/tiflow/blob/5a0452d3fefb157244520a11f21f102de57a1937/dm/syncer/syncer.go#L1332-L1336

to avoid interfering the logic between modification and using. Like https://github.com/pingcap/tiflow/blob/5a0452d3fefb157244520a11f21f102de57a1937/dm/syncer/syncer.go#L1373

2. there're too many warnings in log, we know in most of time there's no timezone information in status vars, so should not raise warning so frequently https://github.com/pingcap/tiflow/blob/5a0452d3fefb157244520a11f21f102de57a1937/dm/syncer/shardddl.go#L158

What did you expect to see?

No response

What did you see instead?

No response

Versions of the cluster

master

current status of DM cluster (execute query-status <task-name> in dmctl)

(paste current status of DM cluster here)