Ensure we don't upgrade systemd to v243+ without fixing the haveged incompatibility

Our options:

Cross fingers and hope we’ll notice if we pull systemd v243+ from backports in Tails 4.x, or when we’ll be preparing 5.0. Hmm…
Add a comment in our apt/preferences, in the place where one would add the systemd section? Not really satisfying but possibly good enough.
Add a build-time hook that says “ensure you also get a fix for #NNNNN and then delete this hook” + exit 1, if systemd >= 243.

kibi and I prefer the last option.

Original created by @intrigeri on 17008 (Redmine)

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