title: Process not defined
body: <p>I created an app with create-react-app without ejected config. 
  When I try to access the process it is always undefined.</p>
  
  <p>I added an .env file in the root folder, with a key <pre>REACT_APP_PREFILL_URL=SOMEURL</pre></p>
  
  <p>After I start yarn or npm start, I get "process is not defined"</p>
  
  <p>Here is my code</p>
  
  <pre>
   return axios({
              method: 'post',
              url: process.env.REACT_APP_PREFILL_URL,
              headers: {
                  'x-api-key': process.env.REACT_APP_PUBLIC_API_KEY,
                  'Cache-Control': 'no-cache, no-store',
                  Pragma: 'no-cache',
              },
              data: {
                  country: 'de',
              },
              timeout: 30000,
              validateStatus: status => status === 200,
          }).then(response => response.data.id);
  </pre>