ci: Use nix+cachix

[justinrubek]

Switches CI actions to use nix, taking advantage of cachix to cache nix derivations.

The built packages and tests are ran inside a nix context using artifacts generated using crane which reduces repeated builds of dependencies that have not changed. This means that the build generated from a CI test run can be used directly to create a container image since it will be cached. This means in most cases (when the only code being changed is the code in this repository) the CI build times should be cut down significantly.

For running tests this uses cargo-nextest. It has been provided in the devshells.

As a result of runny clippy through crane, we are no longer using clippy-check and will no longer have the pretty display on the PR diff view. However the speed increase for CI is substantial in some cases (while remaining on par in others with the existing CI). Here are some example runs:

Action run when nothing needed rebuilt: ~2 minutes
Running tests: ~16 minutes
Building image: ~2 minutes (when the package has been cached due to a previous test)
Clippy failure check: ~9 minutes

From here this is still some potential for speedup in future work. Potentially running the actions on a NixOS host (or, at least a persistent runner) could help cache some dependencies between runs. There also may be other options for configuring the existing system that are not taken advantage of yet.

[justinrubek]

I closed and re-opened the PR to see about the clippy check. It seems that this may be an option set on the repository settings. I can try to tweak this to work with that, or maybe we can just make the CI action be the only thing required (see the diff and commits/action runs to see that clippy is enforced). This would explain #120 as well

[scsmithr]

I had clippy checked as a required action before merging to main. Went ahead and removed it so this should be good to go.