ci: publish a portable ARM64 transaction prover image

[adrienlacombe]

Summary

• publish the linux/arm64 transaction-prover release artifact without a CPU-specific TARGET_CPU
• retain the existing znver5 tuning for the AMD64 artifact

The current ARM64 release is compiled for neoverse-v2, which enables instructions unavailable on Apple Silicon. The published multi-architecture tag selects this true ARM64 image on M3 Docker hosts, where it exits with SIGILL before becoming healthy. Leaving TARGET_CPU empty makes the Dockerfile use Rust's generic AArch64 baseline while preserving the existing ARM64 image and manifest layout.

Validation

• git diff --check
• YAML syntax parse
• confirmed the Dockerfile only sets -C target-cpu=... when TARGET_CPU is non-empty

A native ARM64 startup smoke test can be added separately once an appropriate runner/test RPC fixture is available.

[cursor]

PR Summary

Low Risk
CI-only matrix change with no application logic; may slightly reduce ARM64 proving performance on Neoverse hardware in exchange for broader portability.

Overview
ARM64 release images are no longer built with TARGET_CPU=neoverse-v2. The publish workflow matrix now passes an empty target_cpu for linux/arm64, with a comment that release tags must use the generic AArch64 baseline (including Linux containers on Apple Silicon).

AMD64 is unchanged: the workflow still sets target_cpu: znver5 for linux/amd64. The Dockerfile already skips -C target-cpu when TARGET_CPU is empty, so ARM64 builds use Rust’s default portable AArch64 codegen instead of Neoverse-specific instructions that caused SIGILL on M-series hosts.

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[reviewable-StarkWare]

This change is