Forward registry auth token only if asked

lib/index.js

@@ -64,7 +64,8 @@ export default class Now extends EventEmitter {
     }
 
     // Read .npmrc
-    let npmrc;
+    let npmrc = {};
+    if (pkg['forward-npm']) {
       try {
         npmrc = await readFile(resolvePath(path, '.npmrc'), 'utf8');
         npmrc = parseIni(npmrc);
@@ -80,6 +81,7 @@ export default class Now extends EventEmitter {
           // Do nothing
         }
       }
+    }
 
     const authToken = npmrc['//registry.npmjs.org/:_authToken'];