title: Xcode 10 warning: Stale file '/path/to/file' is located outside of the allowed root paths
body: <p>Xcode 10 produces this warning when running <code>xcodebuild</code> in a run script phase of a build target. I do this to produce the binary for the other platform (simulator/real device) to lipo into a fat binary for distribution. The warning is produced for every file in the target intermediates directory, for example:</p>
  
  <p><code>warning: Stale file '/Users/nick/Library/Developer/Xcode/DerivedData/App-ctjxvxdmomwoobfgvvasfhwvcnhy/Build/Intermediates.noindex/Library-Target.build/Debug-iphonesimulator/Sub-Library-Target.build/Objects-normal/i386/Sub-Library-Class.o' is located outside of the allowed root paths.
  </code></p>
  
  <p>What are the root paths? Why is this warning produced? Is there any way I can resolve or suppress it?</p>