nixos/activation: propagate system to nested configurations
nixos/activation: propagate system to nested configurations

The current behavior lets `system` default to
`builtins.currentSystem`. The system value specified to
`eval-config.nix` has very low precedence, so this should compose
properly.

Fixes #80806

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs