fix: add check if lerna npmClient=yarn

lib/manager/npm/monorepos.js

@@ -65,7 +65,10 @@ async function checkMonorepos(config) {
   if (await platform.getFile('package-lock.json')) {
     logger.debug('lerna has a package-lock.json');
     lernaLockFile = 'npm';
-  } else if (await platform.getFile('yarn.lock')) {
+  } else if (
+    lernaJson.npmClient === 'yarn' &&
+    (await platform.getFile('yarn.lock'))
+  ) {
     logger.debug('lerna has non-workspaces yarn');
     lernaLockFile = 'yarn';
   }