Build output diff

[zimbatm]

When getting a flake update PR for example, it's not obvious what is going to change on the various NixOS system closures.

It would be nice to have an output that shows the diff between the default and current branch.

• nix-diff report for all modified flake packages outputs
• SBOM diff report for all modified flake packages outputs
• nvd/dix report for all modified nixos system in the flake output (Improve nvd/dix report to show systemd services and configuration changes)

/cc @jfroche

[Mic92]

I think the best way to implements is exposing build's constituent that nix-eval-jobs supports because of hydra. This would be a generic way to expose build artifacts per flake attribute.