Set node_version in js-build base job

Defaults in one role do not automatically transfer to
roles via include_role, and applying things correctly
can lead to recursion issues:

  https://github.com/ansible/ansible/issues/36274

Just set the default in the job.

Change-Id: Iabefd80c8fc40a61a601b082f265c93af643cd83

zuul.d/js-jobs.yaml

@@ -37,6 +37,7 @@
     post-run: playbooks/javascript/post.yaml
     vars:
       js_build_command: build
+      node_version: 14
 
 - job:
     name: build-javascript-source-tarball