fix: bundle sfdisk, mkfs.vfat, efibootmgr in initramfs
HD install fails with sfdisk rc=32512 (command not found) because
these disk utilities weren't being copied into the initramfs by
docker-build.sh. The old build-and-flash-initramfs.sh bundled them
but the oven uses docker-build.sh which didn't.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>