[archiso2dual] Set default SquashFS compressor to XZ

Signed-off-by: Gerardo Exequiel Pozzi <vmlinuz386@yahoo.com.ar>

archiso2dual/archiso2dual

@@ -19,7 +19,7 @@ work_dir="/tmp/archiso2dual"
 isomounts_file="/usr/share/archiso2dual/isomounts"
 syslinuxcfg_dir="/usr/share/archiso2dual/syslinux"
 removefiles_file="/usr/share/archiso2dual/removefiles.lst"
-compression="gzip"
+compression="xz"
 install_dir="arch"
 appname=${0##*/}
 _error="0"