nixify ci

[mhuesch]

use https://github.com/cachix/install-nix-action to provision the dev environment (rather than hecrj/setup-rust-action@v1), so that we are dev-ing and CI-test-ing with the same (nix guaranteed) environment.

closes #37.

todo:

• use https://github.com/cachix/cachix-action to cache nix derivations
• look at https://github.com/Swatinem/rust-cache or other Rust caching to speed rust builds