Maybe I got a little bit confused because of the expression “nightly builds”.
Openhabian only uses the stable version at the moment. And the stable version is still hosted on Bintray. Only the unstable version (nightly build, snaphsot version, however it is called…) is now hosted elswere, see here:
You can’t switch from stable to snapshot within openhabian. I think ThomDietrich wants to add this in the future, but atm this isn’t possible.