The user can set "$NMTST_LIBTOOL" to empty or the path to libtool.
If unset, we want to detect it. However, previously we would always
use "$SCRIPT_PATH/../libtool", even if that file doesn't exit. Improve
that.
For example, when running on Ubuntu with the ".gitlab-ci/debian-install.sh"
script, we don't have libtool. Entering such a container and running the
"run-nm-test.sh" script will fail as "$SCRIPT_PATH/../libtool" doesn't
exist.
https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1800