Strip xattrs from squashfs to fix boot failure

Kernel squashfs driver fails with "unable to read xattr id index table"
when the squashfs contains POSIX ACL xattrs (system.posix_acl_*) written
by mksquashfs as unrecognised entries. This caused every built ISO to
drop to an initramfs shell at boot.

Add -no-xattrs to mksquashfs options so xattrs are stripped at build
time. xattrs are not needed in a live read-only rootfs.

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

iso/builder/auto/config

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