Changes

ci(pipelines): back BR integration temp dir with PVC (#4580)

## Summary
- mount `/tmp/tidb` in `pull_br_integration_test` onto a PVC-backed
ephemeral volume
- keep the existing workspace volume unchanged
- initialize the dedicated temp dir permissions in the init container so
TiDB can write DDL ingest data there

## Validation
- `./.ci/verify-k8s-pod-yaml.sh
pipelines/pingcap/tidb/latest/pod-pull_br_integration_test.yaml`
- replay success (full `/tmp` PVC experiment):
https://prow.tidb.net/jenkins/job/pingcap/job/tidb/job/pull_br_integration_test/1015/
- replay success (`/tmp/tidb` PVC, 300Gi):
https://prow.tidb.net/jenkins/job/pingcap/job/tidb/job/pull_br_integration_test/1016/
- replay in progress for final `/tmp/tidb` PVC shape (200Gi):
https://prow.tidb.net/jenkins/job/pingcap/job/tidb/job/pull_br_integration_test/1017/

## Context
`pull_br_integration_test` hit DDL ingest temp-dir pressure under
`/tmp/tidb/tmp_ddl-4000`. This change gives TiDB temp storage its own
PVC-backed space without changing the rest of the pod layout.
 (commit: 211a584)