Migrate to mksquashfs that honors $SOURCE_DATE_EPOCH
On feature/5630-deterministic-builds we currently use a custom
mksquashfs that supports the -mkfs-fixed-time option. But a better
approach would be to make it honor $SOURCE_DATE_EPOCH instead. So we
should replace our patch with the source-date-epoch branch of
https://github.com/lamby/squashfs-tools. And then we can revert
0f56eea534f6cde1bee912cc51ceeb435790df80 in tails.git, and commit
4557f09be29078b4473de48953effec3e956762e in iuk.git.
Feature Branch: feature/5630-deterministic-builds
Parent Task: #5630 (closed)
Original created by @intrigeri on 11976 (Redmine)