Use better xz options

Default

• Build time: 2m29s

• Qemu tcg boot time from boot selection to interactive prompt (stopwatch time): 1m24s

  732M	out/archlinux-2020.11.03-x86_64.iso

With these options:

• Build time: 2m28s

• Qemu tcg boot time from boot selection to interactive prompt (stopwatch time): 1m43s

  675M	out/archlinux-2020.11.03-x86_64.iso

References

Fedora also did some research towards this: https://fedoraproject.org/wiki/Changes/OptimizeSquashFS but they concluded it makes boot too slow: https://meetbot.fedoraproject.org/fedora-meeting-2/2020-09-09/fesco.2020-09-09-14.00.log.html

I argue that in our case, however, 10% slower boot is worth 57MiB.