global sort should not bother to retry when context has error

[lance6716]

Bug Report

tidb/pkg/lightning/backend/external/writer.go

Lines 464 to 473 in 448e302

	for i := 0; i < flushKVsRetryTimes; i++ {
	dataFile, statFile, err = w.flushSortedKVs(ctx)
	if err == nil {
	break
	}
	logger.Warn("flush sorted kv failed",
	zap.Error(err),
	zap.Int("retry-count", i),
	)
	}