Disable xattrs in all mksquashfs calls

--chroot-squashfs-compression-options does not exist in live-build
bookworm (1:20230502). The correct mechanism is the MKSQUASHFS_OPTIONS
environment variable read by binary_rootfs.

Export MKSQUASHFS_OPTIONS="-no-xattrs" before lb build so live-build's
binary_rootfs picks it up, and add -no-xattrs explicitly to every
direct mksquashfs call in build.sh (fast-path repack and the dormant
split-layers function). Remove the invalid lb config option.

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

iso/builder/auto/config

@@ -42,5 +42,4 @@ lb config noauto \
     --debootstrap-options "--include=ca-certificates" \
     --apt-recommends false \
     --chroot-squashfs-compression-type zstd \
-    --chroot-squashfs-compression-options "-no-xattrs" \
     "${@}"