generic-{native,cross}-build: workaround Qt 6.8 in 24.04-1.x break Qbs

I decided to allow Qt 6.8 to enter Ubuntu Touch 24.04-1.x, under the assumption that there's no Qt 6 packages in images yet. However I failed to consider that Clickable does include Qbs from Ubuntu repository and there are library transitions in Debian packaging of Qt 6. The end result is Qbs fails to install.

To workaround this, have an Apt preference file which let system install Qt 6 packages from Ubuntu instead of from Ubuntu Touch.

Note that this also affect 24.04-2.x as well, but we're going to solve this differently.