perf(iso): speed up builds — zstd squashfs + preserve lb chroot cache

- Switch squashfs compression from xz to zstd (3-5x faster compression,
  ~10-15% larger but decompresses faster at boot)
- Stop rm -rf BUILD_WORK_DIR on each build; rsync only config changes
  so lb can reuse its chroot across builds (skips apt install step)
- Keep lb-packages cache in CACHE_ROOT as fallback if work dir is wiped

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/auto/config

@@ -34,4 +34,5 @@ lb config noauto \
     --iso-application "EASY-BEE" \
     --bootappend-live "boot=live components quiet nomodeset video=1920x1080 console=tty0 console=ttyS0,115200n8 loglevel=3 username=bee user-fullname=Bee modprobe.blacklist=nouveau" \
     --apt-recommends false \
+    --compression zstd \
     "${@}"