nixos/sudo-rs: Simplify activation

- [`sudo-rs`], a reimplementation of `sudo` in Rust, is now supported.

  An experimental new module `security.sudo-rs` was added.

  Switching to it (via `security.sudo.enable = false; security.sudo-rs.enable = true;`) introduces

  Switching to it (via ` security.sudo-rs.enable = true;`) introduces

  slight changes in sudo behaviour, due to `sudo-rs`' current limitations:

  - terminfo-related environment variables aren't preserved for `root` and `wheel`;

  - `root` and `wheel` are not given the ability to set (or preserve)

  ###### implementation

  config = mkIf cfg.enable {

    assertions = [ {

      assertion = ! config.security.sudo.enable;

      message = "`security.sudo` and `security.sudo-rs` cannot both be enabled";

    }];

    security.sudo.enable = mkDefault false;

    security.sudo-rs.extraRules =

      let

        defaultRule = { users ? [], groups ? [], opts ? [] }: [ {

          test5 = { isNormalUser = true; };

        };

        security.sudo.enable = false;

        security.sudo-rs = {

          enable = true;

          package = pkgs.sudo-rs;

        noadmin = { isNormalUser = true; };

      };

      security.sudo.enable = false;

      security.sudo-rs = {

        package = pkgs.sudo-rs;

        enable = true;