backfill: force batch bytes limit production value if knob is set

This commit is similar in spirit to f6b7969,
and although I'm not sure whether it matters, it seems nice to apply the
testing knob override everywhere. (I just happened to notice this while
modifying some adjacent code.)

Epic: None

Release note: None

pkg/sql/backfill/backfill.go

@@ -335,7 +335,7 @@ func (cb *ColumnBackfiller) RunColumnBackfillChunk(
 	// read or used
 	if err := cb.fetcher.StartScan(
 		ctx, []roachpb.Span{sp}, nil, /* spanIDs */
-		rowinfra.GetDefaultBatchBytesLimit(false /* forceProductionValue */),
+		rowinfra.GetDefaultBatchBytesLimit(cb.evalCtx.TestingKnobs.ForceProductionValues),
 		chunkSize,
 	); err != nil {
 		log.Errorf(ctx, "scan error: %s", err)
@@ -864,7 +864,7 @@ func (ib *IndexBackfiller) BuildIndexEntriesChunk(
 	defer fetcher.Close(ctx)
 	if err := fetcher.StartScan(
 		ctx, []roachpb.Span{sp}, nil, /* spanIDs */
-		rowinfra.GetDefaultBatchBytesLimit(false /* forceProductionValue */),
+		rowinfra.GetDefaultBatchBytesLimit(ib.evalCtx.TestingKnobs.ForceProductionValues),
 		initBufferSize,
 	); err != nil {
 		log.Errorf(ctx, "scan error: %s", err)