needRescan region job should not pass retryCount to new jobs

[lance6716]

tidb/br/pkg/lightning/backend/local/local.go

Lines 1277 to 1281 in 5c31e16

	// 1 "needRescan" job becomes len(jobs) "regionScanned" jobs.
	jobWg.Add(len(jobs) - 1)
	for _, j := range jobs {
	j.retryCount = job.retryCount
	jobOutCh <- j

In a recent import task we rely on unlimited retry number for this type of error

[lance6716]

also cc @D3Hunter