title: Postcss in Parcel-bundler does not work; No CSS modules used
body: <p>I am using Parcel-bundler@1.12.3, installed with npm.</p>
  <p>My CSS is generic pure CSS with no modules or SASS or anything used.</p>
  <p>The &quot;parcel index.html&quot; works perfectly.</p>
  <p>However, when run &quot;parcel build index.html --no-content-hash --out-dir public --no-cache --no-source-maps --public-url ./&quot;, parcel began to automatically install postcss and all the dependencies. Then throw this error.</p>
  <p>I have tried to disable all my original CSS and use a dummy CSS for my index.html, the issue still persist.</p>
  <p>test.css</p>
  <pre><code>body {
  background-image: linear-gradient(90deg, #303133, #303133);
  font-family: 'Open Sans', sans-serif;
  font-weight: 200;
  min-height: 100vh;
  margin: 0;
  color: #d3d3d3;
  }
  </code></pre>
  <p>Error:</p>
  <blockquote>
  <p>C:\Users...\frontend\styles\test.css:undefined:undefined: plugin is not a function
  at LazyResult.run (C:\Users...\node_modules\parcel-bundler\node_modules\postcss\lib\lazy-result.js:288:14)
  at LazyResult.asyncTick (C:\Users...\node_modules\parcel-bundler\node_modules\postcss\lib\lazy-result.js:212:26)
  at C:...\node_modules\parcel-bundler\node_modules\postcss\lib\lazy-result.js:254:14 at new Promise ()
  at LazyResult.async (C:\Users...\node_modules\parcel-bundler\node_modules\postcss\lib\lazy-result.js:250:23)
  at LazyResult.then (C:\Users...\node_modules\parcel-bundler\node_modules\postcss\lib\lazy-result.js:131:17)</p>
  </blockquote>
  <p><a href="https://i.stack.imgur.com/TEPIG.png" rel="nofollow noreferrer">Error msg in console</a></p>