fix: skip ldid signing on ARM64 — macOS artifacts are only published from amd64 (#22120)

## 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`"

Author: alexghr

barretenberg/cpp/bootstrap.sh

@@ -37,9 +37,11 @@ function inject_version {
   printf "$version\0" | dd of="$binary" bs=1 seek=$version_offset conv=notrunc 2>/dev/null
 
   # Re-sign after modifying the binary.
+  # On macOS, use codesign. On Linux amd64, use ldid for Mach-O cross-compiled binaries.
+  # ARM64 Linux instances don't need ldid — all macOS release artifacts are published from amd64.
   if [[ "$(os)" == "macos" ]]; then
     codesign -s - -f "$binary" 2>/dev/null || true
-  elif llvm-objdump-20 --macho --private-header "$binary" 2>/dev/null | grep -q "Mach header"; then
+  elif [[ "$(arch)" == "amd64" ]] && llvm-objdump-20 --macho --private-header "$binary" 2>/dev/null | grep -q "Mach header"; then
     ldid -S "$binary"
   fi
 }