Changes

fix(tidb/release-8.5): port latest BR prepare flow and remove stage timeout (#4363)

## Summary

Port the `latest` BR pull integration pipeline prepare implementation
into `release-8.5`, then remove an incompatible stage timeout that
caused replay failures on release artifacts.

## Root cause

After porting the `latest` prepare flow, `release-8.5` also inherited:

- `options { timeout(time: 15, unit: 'MINUTES') }` on `Checkout &
Prepare`

For `release-8.5`, artifact download can take longer. During replay,
Jenkins terminated the download step with:

- `script returned exit code 143`
- `Timeout has been exceeded`

## What changed

1. Ported prepare implementation from
`pipelines/pingcap/tidb/latest/pull_br_integration_test.groovy` into
`release-8.5`:
- download components directly under `tidb/bin`
- cache workspace with `ws/${BUILD_TAG}` and `rev-${REFS.pulls[0].sha}`
marker

2. Removed the 15-minute stage timeout from `Checkout & Prepare` in
`release-8.5` to avoid premature termination of artifact download.

## Verification

- Replay reference (passed):

https://do.pingcap.net/jenkins/blue/organizations/jenkins/pingcap%2Ftidb%2Frelease-8.5%2Fpull_br_integration_test/detail/pull_br_integration_test/2037/pipeline/

---------

Signed-off-by: lyb <yebin.li@pingcap.com>
 (commit: 35dedfa)