fix: skip ldid signing on ARM64 — macOS artifacts are only published from amd64

[AztecBot]

Summary

The nightly Docker image build has been failing since March 25 because the ARM64 release instance tries to run ldid (an x86_64 Linux binary) to re-sign cross-compiled Mach-O binaries after version injection.

This was exposed by PR #21953 which fixed llvm-objdump → llvm-objdump-20, making Mach-O detection actually work — before that, the detection silently failed so ldid was never called.

Fix

Gate the ldid call behind $(arch) == amd64 in inject_version. The ARM64 instance doesn't need to sign Mach-O binaries because all macOS release artifacts are published exclusively from the amd64 instance:

• GitHub releases (tarballs): packaged by build_release_dir, runs on amd64 only
• bb.js npm packages: copy_cross.sh already guards ldid with [[ "$(arch)" == "amd64" ]]
• Docker images: Linux binaries only, no Mach-O signing needed

The ARM64 instance builds cross targets (macOS, Windows, iOS, Android) to populate the shared build cache, but never publishes them.

Impact

Unblocks the nightly release pipeline → unblocks Deploy Next Net (failing 6 days).

Changed file

barretenberg/cpp/bootstrap.sh — one-line change in inject_version"