CI: tidb_parser_test fails with 'toolchain not available' for go1.25.8 on linux/amd64

## Bug Report

### 1. Minimal reproduce step (Required)

Submit any PR that triggers the `tidb_parser_test` CI job. Observed on PR #66194.

CI job URL: https://tiprow.hawkingrei.com/view/gs/pingcapprow/pr-logs/pull/pingcap_tidb/66194/tidb_parser_test/2036363138772766720

The runner fails to download the Go toolchain required by `go.toolchain` directive in `go.mod`:

```
go: download go1.25.8 for linux/amd64: toolchain not available
```

This causes the entire `tidb_parser_test` pipeline to fail before any tests run.

### 2. What did you expect to see? (Required)

The CI runner should successfully resolve or use the required Go toolchain and proceed to run parser tests.

### 3. What did you see instead (Required)

The `tidb_parser_test` CI job exits immediately with:

```
go: download go1.25.8 for linux/amd64: toolchain not available
```

No parser tests are executed. The failure is pre-existing and unrelated to any code changes in the triggering PR.

### 4. What is your TiDB version? (Required)

Branch: `master` / `copilot/fix-tikv-region-status`

The CI failure appears to be an infrastructure issue: the `go1.25.8` toolchain binary is not available on the Linux amd64 CI runner environment. This may affect any PR that triggers `tidb_parser_test` on this runner.

**Suggested labels:** `type/bug`, `component/build`, `severity/minor`