piuparts_wrapper: make sure we have the sbin directories in our PATH

Starting with Debian/wheezy /usr/local/sbin, /usr/sbin and /sbin
aren't available inside $PATH of non-root users under sudo if

  Defaults secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"

is not defined in /etc/sudoers.