I'm hitting the same problem.
- Xcode 10 beta installed and selected:
$ xcode-select -p
/Applications/Xcode-beta.app/Contents/Developer
$ xcodebuild -version
Xcode 10.0
Build version 10L176w
- Swift toolchain installed
- tried
utils/build-script,utils/build-script --release-debuginfo,utils/build-script --reconfigure,utils/build-script --clean, all no luck.
Any suggestions?