Needs multi-arch iOS image drop

[Helge Heß]

The stuff currently compiles an image for macOS. Which is great on macOS :-) But the template is for iOS, where you probably want to use this.

So we should adjust this to setup environments and do a build for iOS devices and the simulator. Should not be hard, just invoke w/ the right env settings. Maybe that should be builtin in swift-xcode-image.