title: CircleCI - different value to environment variable according to the branch
body: <p>I'm trying to set different values for environment variables on CircleCI according to the current $CIRCLE_BRANCH.
  I tried setting two different values on CircleCI settings and exporting them accordingly on the deployment phase, but that doesn't work:</p>
  
  <pre><code>deployment:
    release:
      branch: master
      commands:
        ...
        - export API_URL=$RELEASE_API_URL; npm run build
        ...
    staging:
      branch: develop
      commands:
        ...
        - export API_URL=$STAGING_API_URL; npm run build
        ...
  </code></pre>
  
  <p>How could I achieve that?</p>
  
  <p>Thanks in advance.</p>