Changes

ci(prow): stabilize tiflash sanitizer presubmits (#4589)

## Summary

This PR updates the TiFlash sanitizer presubmit jobs in
`prow-jobs/pingcap/tiflash/latest-presubmits.yaml` to make ASAN/TSAN
runs more stable and reduce infra-related failures.

## Changes

- increase sanitizer job resources from `12c/48Gi` to `24c/64Gi`
- tune parallelism:
  - `ASan`: `build_jobs=20`, `test_workers=16`
  - `TSan`: `build_jobs=20`, `test_workers=10`
- use per-job `RUSTUP_HOME` and `CARGO_HOME`
- `unset RUSTUP_TOOLCHAIN` before build
- preinstall the Rust toolchain before `ninja`, with retry on failure
- add `pod_pending_timeout: 30m`

## Why

Recent sanitizer jobs were unstable for infra/runtime reasons before
reaching useful test results:

- Rust toolchain installation could fail when multiple Rust targets
triggered `rustup` during the build
- sanitizer builds, especially ASAN, were too close to the memory limit
- some jobs spent too long in pending state before starting

These changes aim to make sanitizer jobs stability.

## Validation
### on master
-
https://prow.tidb.net/view/gs/prow-tidb-logs/pr-logs/pull/pingcap_tiflash/10835/pull-sanitizer-tsan/2054787523078524928
-
https://prow.tidb.net/view/gs/prow-tidb-logs/pr-logs/pull/pingcap_tiflash/10835/pull-sanitizer-asan/2054539288770514944

<img width="773" height="228" alt="image"
src="https://github.com/user-attachments/assets/46022c65-761b-4444-8f8a-cd3668d17913"
/>

<img width="762" height="256" alt="image"
src="https://github.com/user-attachments/assets/6cc58358-6982-42a0-8380-19f8d56a7218"
/>

### on feature/fts
-
https://prow.tidb.net/view/gs/prow-tidb-logs/pr-logs/pull/pingcap_tiflash/10834/pull-sanitizer-tsan/2054811158065451008
-
https://prow.tidb.net/view/gs/prow-tidb-logs/pr-logs/pull/pingcap_tiflash/10834/pull-sanitizer-asan/2054514829502189568

<img width="815" height="237" alt="image"
src="https://github.com/user-attachments/assets/0c816e6b-f42e-4052-84c5-9583be7f4953"
/>

<img width="821" height="256" alt="image"
src="https://github.com/user-attachments/assets/b4447164-6716-45df-aaff-fd4531444c57"
/>

---------

Signed-off-by: lyb <yebin.li@pingcap.com>
 (commit: 9e5f57b)