preinst is in juju-1-default 1.25.5-0ubuntu3.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 | dpkg-divert --package juju-1-default --add --rename \
--divert /usr/bin/juju.default /usr/bin/juju
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