Make it possible to reproducibly generate IUKs in CI

Originally created by @anonym on #15287 (Redmine)

Since we’ll generate a lot more of IUKs each release uploading them will be painful for RMs with slow internet connections.

For example, the following VM would do:

4 virtual CPUs (so the IUK generation won’t take too long)
1 GB of RAM (I used to generate IUKs in such a VM two years ago but YMMV)
10 GB /tmp for tails-iuk’s needs, and where we store the generated IUKs (before uploading them to rsync.lizard)
Access to Jenkins build artifacts

Feature Branch: feature/15281-single-squashfs-diff

Edited May 15, 2020 by anonym

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information