root.sh: Run `clean` before `dist-upgrade`

This makes sure we don't need even more space for downloaded packages.

root.sh

@@ -318,6 +318,7 @@ function install_pkgs()
         # shellcheck disable=SC2086
         chroot "${basedir}" apt -y -o Debug::pkgProblemResolver=yes install -y --simulate ${packages}
     fi
+    chroot "${basedir}" apt-get clean
     # Get the updates from additional repositories
     chroot "${basedir}" apt-get -y dist-upgrade
 }