feat: support yarn mutex file

Adds yarn mutex file flag to cmd line if set in env

lib/workers/branch/yarn.js

@@ -56,6 +56,10 @@ async function generateLockFile(tmpDir, env) {
     cmd += ' install';
     cmd += ' --ignore-scripts';
     cmd += ' --ignore-engines';
+    cmd += process.env.YARN_MUTEX_FILE
+      ? ` --mutex file:${process.env.YARN_MUTEX_FILE}`
+      : '';
+
     // TODO: Switch to native util.promisify once using only node 8
     ({ stdout, stderr } = await exec(cmd, {
       cwd: tmpDir,