Compress the SquashFS included in IUKs more aggressively (Closes: #7707).

I have tested this to regenerate Tails_i386_1.1~rc1_to_1.1.iuk.

  - current master branch: 246118400 bytes, 426.30s user 33.91s system
  - with this commit: 229642240 bytes, 940.37s user 40.56s system

That is: 9.3% of decrease in size, 120% of increase in build time.

We build an IUK once, and then it's downloaded many times, and takes that much
room on users' devices, so it seems worth it.