title: Change user-defined variable from fastlane
body: <p>I have a user-defined variable in my Xcode project - <code>MY_VARIABLE</code>:<br>
  <a href="https://i.stack.imgur.com/cEiHs.png" rel="noreferrer"><img src="https://i.stack.imgur.com/cEiHs.png" alt="enter image description here"></a>
  I linked <code>MY_VARIABLE</code> also in my .plist file:<br>
  <a href="https://i.stack.imgur.com/233qP.png" rel="noreferrer"><img src="https://i.stack.imgur.com/233qP.png" alt="enter image description here"></a>
  And then I use it in my code:<br>
  <code>NSString *myVariable = [[NSBundle mainBundle] objectForInfoDictionaryKey:@"MY_VARIABLE"];</code></p>
  
  <p>In the fastfile I have my AppStore lane and, only in that case, I would like to change the value of <code>MY_VARIABLE</code>.</p>
  
  <p>I'm currently using:<br>
  <code>ENV["MY_VARIABLE"] = "appStoreValue"</code><br>
  but this doesn't work.</p>