convert CI to use nix

[mhuesch]

currently we use a mix of:

• nix to provision dev environment
• nix to build (via Cargo.nix)
• Github Actions for CI

for implementing #36, it would be significantly easier (and less likely to introduce future bugs/issues) if we ran CI based on the same, single nix derivation set.

this perhaps means we can't use Github Actions - I'm not sure yet. setting up hydra might be nice, tho is a larger task.

[mhuesch]

it seems like Github Actions have changed and perhaps are no longer free? my Actions seem to have been deleted and no longer run on PRs branches.

this might be related to crypto mining abuse on free cloud services? MSFT responding 🤔

feels like more reason to figure out a nix-CI solution instead.

I think I was wrong about the above. I think the actions got dropped when we renamed the org.

anyways, I still would rather use nix/hydra for CI, because it'll be more extensible.

[mhuesch]

thinking to use GitHub Actions for convenience, with https://github.com/cachix/install-nix-action & https://github.com/cachix/cachix-action.