As explained in commit 2f565f6b,
we do need UID/GID stability for this user & group, regardless
of whether we'll provide incremental upgrades to Tails 4.0.

And while I'm at it, let's explain this in a comment in the script
itself, so that one does not need to inspect Git history in order
to understand why we bother setting a fixed UID/GID here. This
should avoid anyone being mislead into thinking we can drop
these static values in the future.

Partially reverts ae104152.