feat: add musl static build for x86_64-unknown-linux-musl

[obazoud]

Summary

• Add a hitch-linux-x64-musl build target (x86_64-unknown-linux-musl, built via cross) to the release matrix, alongside the existing gnu/arm64 builds.
• Statically linked, so it runs on distros with an older glibc (e.g. RHEL 9 ships glibc 2.34, current release binary requires 2.39).
• Not wired into the npm postinstall selection — only attached as an extra GitHub release asset for manual download, matching the ask in the issue.

Closes #6

Test plan

• Ran the build matrix on a fork (workflow_dispatch/branch-trigger only, not included in this diff): all 5 targets succeeded, including hitch-linux-x64-musl.
• Downloaded the resulting artifact and confirmed with file: ELF 64-bit LSB pie executable, x86-64, static-pie linked — no dynamic glibc dependency.
• Maintainer to verify on a real tag push (no docker/CI access from this environment to test cross's musl toolchain end-to-end beyond the above).