stable branch FTBFS due to broken GID pinning

In the build log I see:

07:38:51 groupmod: GID '110' already exists
[...]
07:38:51 +Debian-exim:x:103:110::/var/spool/exim4:/bin/false

So I guess Debian-exim was added and needs to have its GID pinned.

Original created by @anonym on 16780 (Redmine)

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