fix(arm64): override piCore64's BusyBox with host's static busybox

piCore64 v15.0.0 ships BusyBox built with ARM instructions that QEMU virt
cannot emulate even under -cpu max — applets like mkdir, uname, readlink
SIGILL on first invocation (el0_undef in the panic trace). mount works
because piCore's busybox.suid happens to use a different code path.

Fix: when building the arm64 rootfs, replace piCore's bin/busybox and
bin/busybox.suid with /bin/busybox from the build host (Ubuntu's
busybox-static, statically linked, built for generic ARMv8-A).

Also add busybox-static to Dockerfile.builder so the Docker-based build
flow has the same fallback available.

Long-term: source a known-good ARM64 BusyBox build (Alpine, or our own
from upstream BusyBox) so we don't depend on the build host's package
manager. Tracked as future work.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

build/Dockerfile.builder

@@ -40,6 +40,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     apparmor-utils \
     gcc-aarch64-linux-gnu \
     binutils-aarch64-linux-gnu \
+    busybox-static \
     git \
     kpartx \
     unzip \