Changes

ci(pipelines): update tidb ghpr_unit_test image (#4561)

## Summary
- update `pipelines/pingcap/tidb/latest/pod-ghpr_unit_test.yaml`
- switch the TiDB CI image from `go12320241009` to `go12520260122`
- keep this PR scoped to `ghpr_unit_test` only

## Validation
- replayed `ghpr_unit_test` with inline pod yaml and the new image
- success:
https://prow.tidb.net/jenkins/job/pingcap/job/tidb/job/ghpr_unit_test/2792/
- success:
https://prow.tidb.net/jenkins/job/pingcap/job/tidb/job/ghpr_unit_test/2798/

## Notes
- no additional repo-wide verification was run for this one-line pod
image update
 (commit: 0d0a863)