Allow evaluating each agent without growing memory linearly

[domenkozar]

Problem

Once you reach ~25 agents, memory grows to ~10GB to evaluate all the NixOS machines.

That's problematic for local development.

Ideas

• use https://github.com/nix-community/nix-eval-jobs to evaluate top-level attributes where they are agents and then feed the result into deploy.json
• evaluate each agent using a separate cli call and introduce some parallelism like --jobs 4