add workflow for pushing nix flake artifacts to Cachix

[the-mikedavis]

closes #1717

based on https://github.com/cachix/install-nix-action/tree/b2b9fc6cf9ad2446d49c06143ad34b98bf73be99#usage-with-flakes

I grabbed the helix cache on Cachix. I'll need to coordinate with @archseer to get the auth token secret set up in actions.

[the-mikedavis]

(WIP - will add some docs with instructions for using the cache)

I made another cache the-mikedavis-helix-bbdeca1a and pushed the tip of #1659 and this branch, and subsequent runs of nix run are pretty fast. I'll try it out on my laptop to make sure it's not just the outputs being available in my local store.

edit: it downloaded it straight from cachix and was super fast! 🎊

git checkout md-cachix
cachix use the-mikedavis-helix-bbdeca1a
nix run . -- flake.nix

[colemickens]

❤️ 🎊 looks great!

[archseer]

Added the CACHIX_AUTH_TOKEN secret.

[the-mikedavis]

I wonder if this should be enabled on push to master plus pull_request? That might make the cache fill up quickly though 🤔

[archseer]

I'd only do it to pushes to master, was considering if it would make sense to limit it to once per day/some nightly tag even.

[anna328p]

I wonder if this should be enabled on push to master plus pull_request? That might make the cache fill up quickly though thinking

Cachix does automatic garbage collection

[the-mikedavis]

Even with the garbage collection I'm worried the 5GB would fill up too fast with pull_request on. If we have a lot of PRs open and master isn't updated in a while, the master build might be pruned to make space for builds on PRs, right?

OTOH I think it would be useful to cache pull_requests - it would make it very easy to jump between branches when reviewing or trying out new changes.