ci: fix workspace pvc rollout for migrated jobs (#4512)
## Summary - fix the ineffective workspace PVC rollout from `ci#4509` / `ci#4510` - move migrated GCP Jenkins jobs to the Jenkins kubernetes plugin's real `workspaceVolume genericEphemeralVolume(...)` field - remove the dead `workspace-volume` blocks previously added into pod yaml files
## Root cause The replay log for `pingcap/tidb/release-8.5/pull_build` showed the final created pod still had: - `workspace-volume: emptyDir`
Even after `pod-pull_build.yaml` defined `spec.volumes[name=workspace-volume].ephemeral.volumeClaimTemplate`.
The Jenkins kubernetes plugin treats workspace storage as a dedicated `workspaceVolume` field. When that field is not set in the declarative agent, it falls back to `EmptyDirWorkspaceVolume`, so a raw pod-yaml volume named `workspace-volume` does not actually replace the default workspace volume.
## Scope - keep the same rollout scope as the merged phase-1 / phase-2 change - exclude `PingCAP-QE/ci: seed` as before - add effective `workspaceVolume genericEphemeralVolume(...)` to the affected Jenkins pipelines - delete the ineffective `workspace-volume` pod-yaml blocks
## Validation - confirmed the problem from replay log: - https://prow.tidb.net/jenkins/job/pingcap/job/tidb/job/release-8.5/job/pull_build/30 - `111` modified Jenkinsfiles validated successfully via: - `https://prow.tidb.net/jenkins/pipeline-model-converter/validate` - `131` modified pod yaml files parsed successfully with local YAML validation - rechecked cleanup completeness: - no target pod yaml still contains `name: workspace-volume` (commit: aa4a5cc)
