Ensure Node.js version is always explicitly set in CI

[mcous]

Overview

Little housekeeping PR to implement a couple GitHub Actions best practices:

• Use actions/setup-node rather than relying on whatever version of Node.js happens to be installed by default on the runner
  • Also enables caching on npm packages, because it's easy
• Lock non-GitHub actions to specific commits following GH's recommended security practices

Probably affected by #207 due to yaml formatting.

Test plan

Nothing special needed if CI stays green. Default Node.js version may require tweaking

[micheal-parks]

Also something that should be definitely replicated on APP and RDK