Speed up asset compilation on CI

[dacook]

What we should change and why (this is tech debt)

We currently have two types of assets, compiled with Sprockets and Webpacker.

All Webpacker assets are precompiled, while others are compiled on demand (perhaps only a few per spec?). I think we need to gather common time estimates to understand this better:

• Webpacker: x seconds
• Sprockets (all assets): x seconds

Context

Un-compiled sprockets assets caused a flaky spec: openfoodfoundation/openfoodnetwork#12552

Proposed solution

One option is to cache build temp files as referred here:
openfoodfoundation/openfoodnetwork#12580 (comment)
There doesn't seem to be a pre-built solution for this, and we'd need to handle expiry of old unused temp files. So it's probably not worth going down that path.