Inventory/constraints: add id's to inventory constraints to make them more observable

lib/inventory/tests/default.nix

@@ -92,8 +92,8 @@ in
         not_used_machine = builtins.length configs.not_used_machine;
       };
       expected = {
-        client_1_machine = 5;
-        client_2_machine = 5;
+        client_1_machine = 4;
+        client_2_machine = 4;
         not_used_machine = 2;
       };
     };