ci: stop publishing GNU Linux release artifacts

[bolinfest]

Why

We already prefer shipping the MUSL Linux builds, and the in-repo release consumers resolve Linux release assets through the MUSL targets. Keeping the GNU release jobs around adds release time and extra assets without serving the paths we actually publish and consume.

This is also easier to reason about as a standalone change: future work can point back to this PR as the intentional decision to stop publishing x86_64-unknown-linux-gnu and aarch64-unknown-linux-gnu release artifacts.

What changed

• Removed the x86_64-unknown-linux-gnu and aarch64-unknown-linux-gnu entries from the build matrix in .github/workflows/rust-release.yml.
• Added a short comment in that matrix documenting that Linux release artifacts intentionally ship MUSL-linked binaries.

Verification

• Reviewed .github/workflows/rust-release.yml to confirm that the release workflow now only builds Linux release artifacts for x86_64-unknown-linux-musl and aarch64-unknown-linux-musl.