fix: add —ignore-engines to yarn install (#831)

lib/workers/branch/yarn.js

@@ -55,6 +55,7 @@ async function generateLockFile(tmpDir, logger) {
     logger.debug(`Using yarn: ${cmd}`);
     cmd += ' install';
     cmd += ' --ignore-scripts';
+    cmd += ' --ignore-engines';
     // TODO: Switch to native util.promisify once using only node 8
     ({ stdout, stderr } = await exec(cmd, {
       cwd: tmpDir,