Changes

ci(pipelines/pingcap-inc/tidb): migrate private mysql test job to GCP (#4356)

## Summary
- switch release-8.5 private tidb pipelines to use `REFS.repo` instead
of a hard-coded `tidb` directory
- update `pull_mysql_test` to use `pod_label.withCiLabels(...)` and
remove the Jenkins-specific result reporting path
- migrate the mysql test pod off `hub.pingcap.net` to
`ghcr.io/pingcap-qe/ci/jenkins:v2026.1.18-8-gbb5ada9-go1.25` and remove
unused sidecars

## Testing
- not run


Ref #4281
 (commit: 141973e)