nixos/sudo: Check syntax using the configured package

This is preferable even for regular `sudo`, but will ensure the check is useful
when using `sudo-rs` in the future.

Also, dropped antediluvian comment about the syntax check being disabled,
when it was clearly not commented out:
  - introduced in 2007, commit 6d65f0ae03ae14f3e978d89959253d9a8f5e0ec1;
  - reverted in 2014, commit e68a5b265a,
    but without ammending the comments.

nixos/modules/security/sudo.nix

@@ -283,9 +283,7 @@ in
             src = pkgs.writeText "sudoers-in" cfg.configFile;
             preferLocalBuild = true;
           }
-          # Make sure that the sudoers file is syntactically valid.
-          # (currently disabled - NIXOS-66)
-          "${pkgs.buildPackages.sudo}/sbin/visudo -f $src -c && cp $src $out";
+          "${cfg.package}/bin/visudo -f $src -c && cp $src $out";
         mode = "0440";
       };