Upgrade squashfs-tools to 4.5+
Originally created by @intrigeri on #17466 (Redmine)
We currently use a custom squashfs-tools 1:4.4-1+0.tails1 to build IUKs (on the RMs' systems and on Jenkins isobuilders), whose only difference
with 1:4.4-1 is xattr_initialize_header.patch
: avoid randomness in the filesystem caused by uninitialized ‘unused’ field, which was applied upstream
(https://github.com/plougher/squashfs-tools/commit/c570c6188811088b12ffdd9665487a2960c997a0 and is part of 4.5-1+, which is in Debian testing/sid.
Note, in passing: !364 (comment 166207) suggests this is not needed anymore: 1:4.4-2 and 1:4.4-1+0.tails1 created the same IUKs.
As of 2021-08-30, all the dependencies of 1:4.5-2 are in Buster, except libzstd1 which needs to be pulled from buster-backports.
Action needed: ensure all the relevant systems use squashfs-tools 1:4.5-2 or newer.
Sooner or later the package from testing/sid won't install anymore on Buster. We'll see once we're there if we still need/want to support building IUKs on Buster.