Changes

dm: download tikv-worker for next-gen integration tests (#4524)

## Summary
- Download \`tikv-worker\` binary alongside TiKV via
\`--tikv-worker=\${OCI_TAG_TIKV}\`.

## Why

DM integration tests on next-gen TiDB require a full cluster including a
tikv-worker process and a MinIO server that serves DFS storage for TiKV
and IA (ingest) for tikv-worker.

Without tikv-worker, next-gen TiDB's lightning backend cannot complete
ingest backfill (ADD INDEX, IMPORT INTO, etc.), because
\`kerneltype.IsNextGen()\` unconditionally routes region jobs through
\`objStoreRegionJobWorker\` / \`TiKVWorkerURL\` (see
\`pkg/lightning/backend/local/local.go\` in pingcap/tidb).

Current build #31 on the next-gen DM pipeline fails G00 (ha_cases)
because \`ALTER TABLE ... ADD INDEX\` gets stuck at \"write
reorganization\" with \`Put \"http:///write_sst?...\": http: no Host in
request URL\` — the TiDB never had a tikv-worker URL to talk to.

## Follow-up

A follow-up PR to \`pingcap/tiflow\` will update
\`dm/tests/_utils/run_downstream_cluster\` to spin up MinIO + PD
(keyspace pre-alloc) + TiKV (API V2 + TTL + DFS) + tikv-worker + SYSTEM
TiDB + user keyspace TiDB, mirroring the CDC
\`start_tidb_cluster_nextgen\` helper.

## Test plan

- [ ] Trigger the next-gen DM integration pipeline from a tiflow PR once
both this and the tiflow-side changes land.

Ref: \`tests/integration_tests/_utils/start_tidb_cluster_nextgen\` in
pingcap/ticdc.
 (commit: 7998c72)