ac-os: fix initramfs ownership so oven can rebuild
sudo creates root-owned files in initramfs-root. On the next build,
the oven user can't clean/repopulate them, causing the musl linker
to be missing from the initramfs. chown back to the calling user
after repacking.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>