title: set proxy in package.json to env variable
body: <p>I need to set the <code>proxy</code> value in my <code>package.json</code> at runtime, like with an environment variable. How could this be done? </p>
  
  <pre><code>// package.json
  {
    "name": "demo",
    "proxy": process.env.MY_PROXY_VAL , // &lt;- how?
    "dependencies": {},
    "scripts": {},
  }
  </code></pre>
  
  <p>Thanks.</p>