title: Is there a way to set -UseModernBuildSystem=NO in a Fastlane fastfile?
body: <p>When building my app using <code>fastlane gym</code> I'm getting <code>error: Unexpected duplicate tasks:</code>. When I run into this issue using Xcode I can get rid of it by setting the build system to the legacy build system. I would like to be able to set the build system using fastlane but have not found the correct way to pass it in using <code>xcarg</code>. </p>
  
  <p>I've tried using this command. <code>fastlane gym --xcargs "UseModernBuildSystem=no"</code></p>
  
  <p>Which in turn runs this: <code>set -o pipefail &amp;&amp; xcodebuild -workspace ./PolyAcademy.xcworkspace -scheme PolyAcademy -destination 'generic/platform=iOS' -archivePath /Users/mattmarshall/Library/Developer/Xcode/Archives/2019-10-21/PolyAcademy\ 2019-10-21\ 16.21.58.xcarchive UseModernBuildSystem=no archive | tee /Users/mattmarshall/Library/Logs/gym/PolyAcademy-PolyAcademy.log | xcpretty</code></p>
  
  <p>I'd like for this not to appear and for the app to build.</p>
  
  <pre><code>[16:22:00]: ▸ 2019-10-21 16:22:00.145 xcodebuild[32252:315891]  DTDeviceKit: deviceType from 870b9074181ce2e0318a5477d3bd3536633ee1ee was NULL
  [16:22:01]: ▸ ❌  error: Unexpected duplicate tasks:
  [16:22:01]: ▸ ** ARCHIVE FAILED **```
  </code></pre>