fix(bootstrap): handle existing Rust installations on pacman systems

[Axiaobo7788]

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Problem

install-deps.sh unconditionally installs rustup on pacman systems.
On Arch-derived distributions, rustup conflicts with the distro-provided
rust package, causing make bootstrap-native to abort when Rust is already
installed.

The existing install_rust() check also only verifies that the cargo
command exists. A rustup installation without an installed toolchain may
provide the command while Cargo itself is not usable.

Solution

• Preserve an existing working distro-provided Rust/Cargo toolchain.
• Install rustup on pacman systems only when neither working Cargo nor
  rustup is available.
• Check cargo --version instead of only checking command presence.
• Initialize the stable toolchain when rustup exists but Cargo is not usable.

Validation

Tested on Manjaro x86_64 with the distro rust package installed.

Before:
make bootstrap-native failed because pacman tried to install conflicting
rustup.

After:
scripts/install-deps.sh preserves the existing Rust toolchain and completes
successfully.

Also validated with:

bash -n scripts/install-deps.sh
git diff --check

Checklist

• This pull request is ready for review and is no longer a draft.
• I followed CONTRIBUTING.md, kept the change focused, edited source files rather than generated output, and removed unrelated changes.
• If this fixes upstream drift, it targets only the latest signed stable OpenAI Linux package and removes obsolete fallback code and tests.
• I added or updated relevant tests, ran the validation listed above, and confirmed that required CI checks pass.
• I reviewed the final diff with my coding agent using maximum reasoning effort, addressed all findings, and reran the relevant tests.

[ilysenko]

Reviewed the complete base-to-head diff. The pacman Rust bootstrap fix is focused, preserves existing working toolchains, and is covered by the live Rust-state matrix. No blockers found.