Modernize GitHub Actions workflows

[tisonkun]

This closes #147.

Summary

• upgrade actions/checkout from v4 to v7 and actions/cache from v4 to v6
• replace the archived actions-rs/toolchain and actions-rs/cargo actions with dtolnay/rust-toolchain and direct Cargo/Cross commands
• fix the two failing i686 clang-cl jobs by using the LLVM toolchain bundled with the Windows runner instead of the removed MSYS2 mingw-w64-i686-clang package
• remove the deprecated MSYS2 MINGW32/MINGW64 dependency from Windows GNU testing by using the matching Rust GNU host toolchains
• add read-only token permissions, stale-run cancellation, consistent timeouts, and a working llvm-mingw cache

Failure analysis

Both failures in run 30741090730 stop in msys2/setup-msys2 with error: target not found: mingw-w64-i686-clang. MSYS2 phased out its 32-bit environments and removed CLANG32, so the old C:/msys64/mingw32/bin/clang-cl.exe setup is no longer maintainable.

The replacement uses C:/Program Files/LLVM/bin/clang-cl.exe, which is provided by the GitHub-hosted Windows image and supports both MSVC targets. Windows GNU jobs now install the target-specific Rust host toolchain, including the official rust-mingw component, instead of relying on legacy MSYS2 environments.

Validation

• go run github.com/rhysd/actionlint/cmd/actionlint@v1.7.12 -color .github/workflows/test.yml
• cargo test --all-targets
• cargo test --manifest-path psm/Cargo.toml --all-targets
• git diff --check
• GitHub Actions run 30741670026: all 201 jobs passed

[tisonkun]

I'm reviewing the output and would mark it as ready for review when I'm OK with the PR.

[tisonkun]

@nagisa Thanks for your review! I'll handle this PR today or tomorrow. It is now purely AI generated but I'd pefer to create a draft PR for better human review (by myself) experience.

This should be what a draft PR designed for, though.

[nagisa]

sure, doesn't hurt to give it a quick look and adjust the direction early on though ^^

[tisonkun]

@nagisa This PR should be now reviewable.

[tisonkun]

I checked the AI generated patch and addressed the comments. Briefly, it

1. Mechanically upgrade actions and replace deprecated actions with identical cargo commands calls.
2. Fixup Windows related test matrix by catching up upstream's naming changes.