Dedicated binary cache.

[max-niederman]

At the moment we're just using Garnix's public cache, which isn't great because

1. It has unspecified retention periods.
2. It's really quite slow, at the moment.
3. Maybe some users would trust our cache integrity but not Garnix's?

Additionally, it would probably be good to switch away from Garnix anyway because they're going to start charging for builds soon and I'm not sure whether this project will fit on my personal free tier account.

Possibly GitHub Actions plus Cachix's free tier would be sufficient?