nixos/nixos-containers: actually eliminate costs if no containers are used (#427001)

  and called `setup.py` from the source tree, which is deprecated.

  The modern alternative is to configure `pyproject = true` with `build-system = [ setuptools ]`.

- `boot.enableContainers` is only turned on when a declarative NixOS container is defined in `containers`.

  If you use the `nixos-container` tool for imperative container management, set `boot.enableContainers = true;` explicitly.

### Deprecations {#sec-nixpkgs-release-25.11-lib-deprecations}

- Create the first release note entry in this section!

    boot.enableContainers = mkOption {

      type = types.bool;

      default = true;

      default = config.containers != { };

      defaultText = lib.literalExpression "config.containers != { }";

      description = ''

        Whether to enable support for NixOS containers. Defaults to true

        (at no cost if containers are not actually used).

        Whether to enable support for NixOS containers.

      '';

    };