Variable ember-cli asset fingerprint prepend value

I am trying to setup a cdn url in ember-cli, however I want the actual url to vary between environments.

// ember-cli-build.js

 var app = new EmberApp({
   fingerprint: {
     prepend: 'http://ift.tt/1Jx8YNI' // want this to be variable between envs
     // prepend: 'https://' + process.env.ASSET_HOST + '/' <- this is what I want
   }
 });

I tried adding an environment variable to process.env, but that doesn't appear to exist the way it does if I were to access it on config/environment.js.

Is there a way to have a variable like this in my ember-cli-build.js file?