Changes

refactor(pipelines/pingcap/tidb): use ephemeral PVC workspace volume for release-8.5 pull_build (#4509)

## Summary
- override the Jenkins kubernetes default workspace volume for
`pingcap/tidb/release-8.5/pull_build`
- keep the pod definition in YAML instead of configuring
`workspaceVolume` in the pipeline DSL
- define `spec.volumes[].name=workspace-volume` as an ephemeral
`volumeClaimTemplate`
- keep this PR scoped to a single high-IO migrated GCP job for phase-1
validation

## Why
Migrated Jenkins jobs on GCP currently rely on the kubernetes plugin
default workspace volume when `workspaceVolume` is not set explicitly.
For large build jobs, that means the workspace lives on node-local
ephemeral storage and can be constrained by small-space nodes.

For declarative Jenkins pipelines in this repo, keeping pod
configuration in the pod YAML is the better maintenance model. This PR
therefore validates the rollout pattern by overriding `workspace-volume`
directly in the pod template YAML for one representative job first.

## Validation
- validated `pipelines/pingcap/tidb/release-8.5/pull_build.groovy` via
`.ci/verify-jenkins-pipeline-file.sh`
- parsed `pipelines/pingcap/tidb/release-8.5/pod-pull_build.yaml`
locally and verified `workspace-volume` resolves to an ephemeral PVC
with `storageClassName=hyperdisk-rwo` and `storage=150Gi`
- verified the pod YAML still survives the same annotation-style
round-trip that `pod_label.withCiLabels(...)` performs
 (commit: 0767f14)