[Qt 6 UT only] have a Qt 6 build for libs, then build plugins in qt6-sensor

sensorfw plugin in qt6-sensor has never been removed, but is currently in limbo state citing that Qt 6 SensorFW is not a thing. But last time I checked it is now a thing, so we should have a Qt 6 build of SensorFW for libraries, then build SensorFW plugin in qt6-sensor.

Alternative to this task is to make SensorFW exposes iio-sensor-proxy's DBus interface (or have an adapter). But I reckon that would be more complicated.