roachtest: use distributed merge for index backfill perf test #159414
Description
We have two workloads that measure the performance of index backfill:
schemachange/bulkingest/nodes=12/rows=4000000000/add_column
schemachange/bulkingest/nodes=12/rows=4000000000/create_index
These should use the distributed merge pipeline now. We should update the existing test so that we can see the performance gain (?) in the roachperf dashboard.
Jira issue: CRDB-57834
Epic CRDB-48845