logger: sanitize $PATH before system() to avoid Perl error

Luca Boccassirequested to merge
bluca/adduser:logger_error into master

To reproduce the issue, change the system so that a log message is produced. For example, delete/move away /etc/adduser.conf and try to add a user in a chroot:

root@sid:~# adduser test
warn: `/etc/adduser.conf' does not exist. Using defaults.
Insecure $ENV{PATH} while running with -T switch at /usr/share/perl5/Debian/AdduserLogging.pm line 162.
root@sid:~# echo $?
25

Set $PATH to /usr/bin:/usr/sbin to avoid this error.

Closes: #1108017

Edited by Luca Boccassi

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