Modify app env vars

[limouren]

It's pretty useful in CI / CD. e.g. Delivering a js bundle with hashed name and the deployed plugin needs to know the url to that js bundle.

[rickmak]

This issue is blocking a skygear.io deployment to integrate with continuous integration pipeline.

[cheungpat]

Sorry this is blocking your deployment. Perhaps you can try adding a command which does this? The controller.js binding should be able to support that, so all it takes is add a sub-command.