build: use gnullvm for Windows Rust exec tools

[anp-oai]

Why

#29143 provides a pinned LLVM/MinGW toolchain, but Windows Bazel lint actions could still select the generic MSVC nightly for exec-side Rust tools or resolve host tools from the runner. This is the first step toward making Windows Bazel compile, proc-macro, and build-script actions hermetic.

Parent PR: #29143.

What

• Replace the default Windows nightly registration with x86_64-pc-windows-gnullvm for both exec and target configurations, including the pinned rustc-dev components used by the lint driver.
• Link gnullvm Rust tools through the selected hermetic Clang/MinGW toolchain. Retained MSVC declarations expose Rust's downloaded rust-lld as lld-link.exe instead of searching PATH.
• Couple build-script producers, wrappers, and runners to the same Rust-capable execution platform, then restrict every Windows gnullvm action to local execution while eligible V8 generator, Rust host-tool, and native compilation work runs on Linux RBE with a separate action environment. Windows V8 snapshot generation remains local because it executes a target PE binary.
• Remove the lint lane's forced MSVC host platform and forced Rust-linker preference, and make the V8 Python generator follow Bazel's exec transition.

Validation

• Queried the generated nightly repositories to verify that the gnullvm toolchain contains rustc-dev and replaces the competing x64 MSVC registration.
• Inspected focused aquery output to verify gnullvm rustc, the hermetic Clang driver, MinGW/UCRT/CRT inputs, matched build-script producer/runner platforms, native Windows Rust/lint execution, and Linux-only V8 host-tool/native execution.
• Ran just bazel-lock-check and the Bazel wrapper's Python test suite.

[anp-oai]

@codex review

[chatgpt-codex-connector]

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Reviewed commit: 5ef24a6143

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[anp-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep them coming!

Reviewed commit: 871d8689fd

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

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[anp-oai]

going to try this from the top, fixing v8 first