Find out what's going on with Exim in our ISO build process

The Debian-exim user has been involved in the two cases where we had UID/GID variation accross releases, which broke automatic upgrades. I think we could do something cheap in order to decrease the chances such problems happen.

Related issues

Related to #15419 (closed)
Related to #15690 (closed)
Blocks #15334 (closed)

Original created by @intrigeri on 15418 (Redmine)

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