devel/qt6-base: Fix configure on live systems

Configure will fail on live systems that have sysutils/lttng-ust
installed. Disable detection of LTTngUST to remedy this in the
meantime. I don't think we really need this, though, and it's probably
best not to rely on an unmaintained port.

PR: 280007
Reported by: O. Hartmann <ohartmann@walstatt.org>