When using Xcode 10 and the new build system, the app build fails because the <project>/build/iphone/build directory is generated by our CLI instead of the actual xcodebuild command. While this was okay in the past, the new build system complains about that.
I filed a radar at https://openradar.appspot.com/40906897 and talked to the Apple engineers during WWDC. They assume it to be an issue in the new build system, but we can actually fix it on our end by now pre-creating the directory and let xcodebuild create it. I tested it as a local change and it works with both Xcode 10 and 9.