title: How to interpolate environment variables within netlify.toml config
body: <p>I want to proxy to a different api depending on the environment - I've tried a few variations on the following theme without any luck. What's the correct way of doing this, if its even possible?</p>
  
  <pre><code>[build.environment]
    API_URI="https://dev-api.foo.com/:splat"
  
  [context.production.environment]
    API_URI="https://prod-api.foo.com/:splat"
  
  [[redirects]]
    from = "/api/*"
    to = "$API_URI"
    status = 200
    force = true
  </code></pre>
  
  <p>This does not work.</p>
  
  <p>Although the above config works when I hardcode a URI into the <code>to</code> field, it just fails when I try to interpolate an env var.</p>