Add Nix CI workflow

[rszamszur]

• I have searched the issues of this repo and believe that this is not a duplicate.

Feature Request

I need to play a bit with nix in GH actions prior deciding whether to implement this.

Clarify/test:

• nix setup in actions
• Steps needed to refactor current actions
• /nix/store caching?

Resources:

• https://github.com/cachix/install-nix-action
• https://github.com/cachix/cachix-action

[rszamszur]

The initial try (https://github.com/rszamszur/fastapi-mvc/actions/runs/3044476889) worked surprisingly well (first try 🚀).

The biggest overhead is at:

• Building custom Nix expressions (but this will be done once, and afterward, it will be fetched from Cachix <3)
• Overall time of fetching nix binaries from cache (both nixos and Cachix). Since each job is a a separate VM runner. Now, number of jobs times matrics executions, and the fetch overhead gets heavy.
• A bit of delay on spawning nix-shell (?)

To minimalize fetch overhead, one could:

• Merge all python version-specific jobs into one. Ex: (metrics 3.9, unit tests 3.9, integration tests 3.9). However, I quite like the granularity of separate jobs. One can see at a quick glance what failed. Moreover, this would require some additional configuration/logic to say what stages are allowed to fail (for instance, if some metrics fail, proceed with the rest of the checks, so at the end, there will be a complete picture of what is wrong).
• Use GitHub cache for caching /nix/store (although I don't think this will be that much faster)
• ?

[rszamszur]

After extensive experiments with the current GH action automation, I've decided that the nix CI workflow will be an additional one instead of refactoring. Therefore it can take a bit different implementation, which should resolve the above-mentioned pinpoints. Moreover, having two similar CI workflows alongside can give a good overview of their performance, issues, and reproducibility, to see what's better or verify if Nix expressions are working as expected.

[rszamszur]

Because of: #146. Changes regarding the project template will be done already in its separate repository.