In my tests on current experimental branch, this makes the ISO 10% smaller, but
makes the build take 63% more (real) time, and eat 110% more (user) CPU time.
It seems worth it for released ISO images.

Likely, most developers that didn't do it yet will now set
MKSQUASHFS_OPTIONS='-comp gzip' when doing their regular builds... although the
time gained by gzip (vs. aggressive xz) at build time is compensated, in many
cases, by faster IO (e.g. when copying the ISO out of the build VM, when booting
the ISO, when installing the ISO on a USB stick).