CI: Fix ARM Linux (aarch64-unknown-linux-musl) build

[jonesd]

Problem

The aarch64-unknown-linux-musl release build fails at the "Install aarch64 musl cross-compilation tools" step. The wget for https://musl.cc/aarch64-linux-musl-cross.tgz fails with exit code 4 (network/download error).

This is marked continue_on_error: true in the release workflow, so it does not block the release — but ARM Linux binaries are not produced.

File

.github/workflows/release.yml:134-140

Possible fixes

1. Use cross tool instead of manual cross-compiler download
2. Use a different mirror or retry logic for the toolchain download
3. Use a Docker-based cross-compilation approach
4. Cache the cross-compiler toolchain between runs

Context

Previously tracked in TODO.md. The Windows build was fixed with #[cfg(unix)] guards. ARM remains the only failing platform.