Thanks for the quick answer 
You were right, they have done an awesome job at hiding the build output panel 
The problem was that xcode could not find my installation of mix (which is installed through nix). After specifying the whole path the script went through and the app “started” (as in: it did show the launch screen).
Now I have the problem that it crashes on start but I assume that this is because of non matching BEAM versions since I haven’t yet installed the correct elixir Version specified in the readme.
