Cache Node.js modules in CI

Looking at https://buildkite.com/automattic/simplenote-electron/builds/299, I noticed we still have no cache for the Node.js modules.

See also implementation in https://buildkite.com/automattic/simplenote-electron/builds/299/canvas?sid=01955a53-0fa7-4a77-8e69-c0dcdbb4d93a

Start by reviewing Buildkite build 299 and its canvas, linked in the issue, to understand the existing CI setup and the referenced implementation. Locate the repository's CI configuration and add module caching there; done means later builds reuse the Node.js module cache and the build completes successfully.

Written by the indexing model from the issue text.