AzureMonitor: Correctly update subscriptions value in ARG editor (#55860)

* Correctly update subscriptions prop in query

* Simplify setting of subscriptions value

public/app/plugins/datasource/grafana-azure-monitor-datasource/components/SubscriptionField.tsx

@@ -56,9 +56,10 @@ const SubscriptionField: React.FC<SubscriptionFieldProps> = ({
         return;
       }
 
-      query.subscriptions = change.map((c) => c.value ?? '');
-
-      onQueryChange(query);
+      onQueryChange({
+        ...query,
+        subscriptions: change.map((c) => c.value ?? ''),
+      });
     },
     [query, onQueryChange]
   );