IUK generation: stop using the "extreme" extra compression option

As my tests show:

• On lizard, creating IUKs on this branch takes only 5% more time than with mksquashfs, and 30% less time than with the extreme compression option.
• The resulting IUKs are a tiny bit smaller than those generated by mksquashfs, and the same size as those we get with the much slower extreme compression option.

QA:

• Test reproducibility
  • 3 IUKs built on intrigeri's laptop (Tails_amd64_4.21_to_4.24.iuk, Tails_amd64_4.22_to_4.24.iuk, and Tails_amd64_4.23_to_4.24.iuk) match those built on Jenkins

Closes #18675 (closed)