ci: run Bazel clippy on Windows gnullvm (#16067)

## Why

We want more of the pre-merge Rust signal to come from `bazel.yml`,
especially on Windows. The Bazel test workflow already exercises
`x86_64-pc-windows-gnullvm`, but the Bazel clippy job still only ran on
Linux x64 and macOS arm64. That left a gap where Windows-only Bazel lint
breakages could slip through until the Cargo-based workflow ran.

This change keeps the fix narrow. Rather than expanding the Bazel clippy
target set or changing the shared setup logic, it extends the existing
clippy matrix to the same Windows GNU toolchain that the Bazel test job
already uses.

## What Changed

- add `windows-latest` / `x86_64-pc-windows-gnullvm` to the `clippy` job
matrix in `.github/workflows/bazel.yml`
- update the nearby workflow comment to explain that the goal is to get
Bazel-native Windows lint coverage on the same toolchain as the Bazel
test lane
- leave the Bazel clippy scope unchanged at `//codex-rs/...
-//codex-rs/v8-poc:all`

## Verification

- parsed `.github/workflows/bazel.yml` successfully with Ruby
`YAML.load_file`