Cache generate-assets output in CI

[BD103]

Closes #1066.

This adds a cache for generate-assets, which is the CI job currently takes the longest at ~7 minutes, and reduces its runtime to 11 seconds. The cache is updated ever time the website is deployed using deploy.yml. The cache is then used in ci.yml when no run in the merge queue.

[TrialDragon]

It looks good from the workflow. I'm still a bit hesitant since we can't test saving the cache until deployment.

[BD103]

One thing I do need to check is whether actions/cache/save overwrites the previous cache of the same key. If it doesn't, I'll need to use the Github API to delete the old cache before creating a new one. (And that could lead to data races with multiple runners, if not handled properly.)

Edit:

This is in fact an issue. Caches cannot be overwritten once created.

I'm going to fix it by adding the date to the cache id, so it updates weekly.

[BD103]

We did some testing (thanks @TrialDragon!). Saving and using the cache worked well, but we were unable to test using it with merge queues because it is a feature restricted to Github orgs and private repos.

Another note: using the cache reduces generate-assets times to 11 seconds!