Summary:
React Native needs to be updated to support XCode 10 (GitHub issue: [19573](https://github.com/facebook/react-nåative/issues/19573)). Until we make that change, our CI needs to rely on the legacy build system in XCode 10 to build `RNTester`, as opposed to the new build system made default in XCode 10. This diff uses the `-UseModernBuildSystem=NO` `xcodebuild` flag to enforce that requirement.
**Note:** I did a search within `react-native-github/scripts` for `xcodebuild` and passed in the `-UseModernBuildSystem=NO` flag. I figured that every time we use `xcodebuild`, we should enforce the build system.
� This should hopefully fix the `react-native-oss-ios` test.
Reviewed By: hramos
Differential Revision: D10456139
fbshipit-source-id: 795f32b3ceba6a5a24ab6ccbf4f5160d42746801