Bug#763169: apt upgrade fails in postinst with "adduser: not found"

[Max Kellermann <max@duempel.org>]

Package: apt
Version: 1.1~exp3

During upgrade:

Setting up apt (1.1~exp3) ...
/var/lib/dpkg/info/apt.postinst: 30: /var/lib/dpkg/info/apt.postinst: adduser: not found
chown: invalid user: '_apt:root'
dpkg: error processing package apt (--configure):
 subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
 apt
E: Sub-process /usr/bin/dpkg returned an error code (1)
Failed to perform requested operation on package.  Trying to recover:
Setting up apt (1.1~exp3) ...
/var/lib/dpkg/info/apt.postinst: 30: /var/lib/dpkg/info/apt.postinst: adduser: not found
chown: invalid user: '_apt:root'
dpkg: error processing package apt (--configure):
 subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
 apt