Building image fails on btrfs due to mkfs.erofs xattr limitation

Unless extended attributes are disabled (with -x-1), mkfs.erofs fails when the source is a BTRFS filesystem. This prevents the Spectrum image from being built. Using -x-1 seems to produce a working installer, though I only smoke tested it (by trying the example VMs). -- Sincerely, Demi Marie Obenour (she/her/hers)