ci: stop publishing GNU Linux release artifacts (#19445)

## 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`.

.github/workflows/rust-release.yml

@@ -69,14 +69,11 @@ jobs:
             target: aarch64-apple-darwin
           - runner: macos-15-xlarge
             target: x86_64-apple-darwin
+          # Release artifacts intentionally ship MUSL-linked Linux binaries.
           - runner: ubuntu-24.04
             target: x86_64-unknown-linux-musl
-          - runner: ubuntu-24.04
-            target: x86_64-unknown-linux-gnu
           - runner: ubuntu-24.04-arm
             target: aarch64-unknown-linux-musl
-          - runner: ubuntu-24.04-arm
-            target: aarch64-unknown-linux-gnu
 
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6