Stop preboot plugins only once on stop() (#171170)

## Summary

When running `setup()` followed by `stop()`, the preboot plugins can ATM
be stopped multiple times.

packages/core/plugins/core-plugins-server-internal/src/plugins_service.ts

@@ -201,7 +201,7 @@ export class PluginsService
     this.log.debug('Stopping plugins service');
 
     if (!this.arePrebootPluginsStopped) {
-      this.arePrebootPluginsStopped = false;
+      this.arePrebootPluginsStopped = true;
       await this.prebootPluginsSystem.stopPlugins();
     }