Draft: Builds APKs with Qt 6 on invent
This is basically the same as !513 (merged) but for master
Reason for the change
We want to retire Binary Factory. Therefore we migrate building of APKs to invent.
Test plan
The APKs will hopefully built against Qt 6. (On Binary Factory the build seems to be broken since the switch to Qt 6.) The APKs may not yet work though. At least, Volker mentioned that the KTrip Qt 6 APK doesn't work yet, but that's unrelated to whether the APKs are built on Binary Factory or on invent. You can download the built APKs and test them. The APKs are not signed because we only sign APKs on release branches and on master.