fix(yarn): strip frozen-lockfile from yarnrc

Closes https://github.com/renovatebot/config-help/issues/195

lib/manager/npm/post-update/index.js

@@ -189,6 +189,7 @@ async function writeExistingFiles(config, packageFiles) {
         upath.join(basedir, '.yarnrc'),
         packageFile.yarnrc
           .replace('--install.pure-lockfile true', '')
+          .replace('--install.frozen-lockfile true', '')
           .replace(/^yarn-path.*$/m, '')
       );
     }