Speed-Up CI

[kjvalencik]

This makes a few changes to our CI process to make it run faster.

• Adds caching of artifacts (Rust, NPM, Electron)
• Collapses to a single CI file for tests to minimize duplication
• Only run on Node LTS and Rust Stable in PRs
  • Runs the full matrix if the "full matrix" label is provided
  • Runs the full matrix on Linux
• Runs the full matrix when merged

Try changing the labels on the PR to test it out!

Note: Matrix includes do not include arrays for a 2nd pass of expansion; instead they become literal arrays. As a workaround, I included a JavaScript snippet in the setup step that performs a manual expansion of the matrix.

[kjvalencik]

@dherman You can write a custom action and include it with your project, giving a path for the action. The action can then invoke other actions. AFAIK, this is the only way to share a snippet which I would consider too complicated for the amount of duplication here.

If these eventually get much more complicated, we could consider that.