Improve github actions efficiency

[GlitchWitch]

Currently both the npm install and compressed images are not cached on GH actions, this should be changed to reduce build time further.

Caching ruby gems for bundler shaved 2 minutes off the build time (from 5:30 to 3:30)

[GlitchWitch]

It might make sense to add these improvements to GlitchSecure/jekyll4-deploy-gh-pages

[GlitchWitch]

We have since switched to Cloudflare pages.

Pages is currently beta testing a cache feature, in the meantime it may make sense to simply pre-run the image compression on all existing images to reduce this as a quick fix.