title: Run npm script with environment variable from package.json
body: <p>I am using a specific task runner to execute my node scripts called Yoshi. I want to run a script from <code>package.json</code>, for example, <code>yoshi test</code> but with a <strong>custom environment variable</strong> (for example <em>special_variable</em>):</p>
  
  <pre class="lang-json prettyprint-override"><code>"scripts": {
      "start": "yoshi start", 
      "test":  "special_variable=value yoshi test // this case"
  }
  </code></pre>