Changes

ci: temporary go1.25.8 toolchain pin for tiflow/ticdc unit jobs (#4366)

## Summary
This PR adds a **temporary CI-side workaround** for mixed Go patch
toolchain artifacts (`go1.25.8` vs older `go tool`) that cause compile
failures in PR jobs.

## Why
Some jobs are still running in environments/images that are not fully
aligned to Go `1.25.8`, while repo-side changes already pull `1.25.8`
artifacts. This can trigger errors like:

```text
compile: version "go1.25.8" does not match go tool version "go1.25.x"
```

## Changes
- `pipelines/pingcap/tiflow/latest/ghpr_verify.groovy`
  - export `GOTOOLCHAIN=go1.25.8`
  - set per-command `GOCACHE`
  - run `go clean -cache` before `make`
  - add explicit TEMP/TODO comments

- `prow-jobs/pingcap/ticdc/latest-presubmits.yaml`
- `prow-jobs/pingcap/ticdc/latest-presubmits-next-gen.yaml`
- same temporary env/cache workaround for `pull-unit-test` and
`pull-unit-test-next-gen`

## Notes
- This is intentionally temporary and should be **reverted** after the
corresponding CI images/environments are fully upgraded and stable on Go
`1.25.8`.
- This does not change product code or test logic; it only stabilizes CI
execution environment consistency.
 (commit: 6028f43)