ci: publish a native furrow-linux-arm64 binary

[santoshkumarradha]

Summary

v0.1.0 ships furrow-linux-amd64, furrow-darwin-arm64, and furrow-darwin-amd64 — arm64 Linux is the one mainstream target with no asset. That means DGX Spark and Jetson, Graviton, Ampere, and Raspberry Pi users must install a Rust toolchain and build from source before they can run furrow at all, including on the far side of an ssh:// remote where the client invokes a bare furrow __remote.

This adds one matrix entry. Built natively on a hosted ubuntu-24.04-arm runner rather than cross-compiled, so it is covered by the existing --version smoke test (native: true) instead of shipping unverified the way furrow-darwin-amd64 necessarily does.

No other workflow logic changes: staging, checksums, and publish already iterate over whatever the matrix produces.

Test plan

• CI green on this PR
• After merge, run the release workflow via workflow_dispatch against v0.1.0 — it uploads with --clobber, so it adds furrow-linux-arm64 to the existing release without disturbing the other three assets or creating a new tag
• furrow --version on an aarch64 Linux box from the published asset
• SHA256SUMS lists all four binaries

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