MM-12974: guard app plugins with mutex (#9818)

* guard app plugins with mutex

Shutting down the app could race with a goroutine that uses the plugins environment, since we shut down the plugins first before cleaning up goroutines.

* fix go vet issues

app/plugin_hooks_test.go

@@ -44,7 +44,7 @@ func SetAppEnvironmentWithPlugins(t *testing.T, pluginCode []string, app *App, a
 	env, err := plugin.NewEnvironment(apiFunc, pluginDir, webappPluginDir, app.Log)
 	require.NoError(t, err)
 
-	app.Srv.Plugins = env
+	app.SetPluginsEnvironment(env)
 	pluginIds := []string{}
 	activationErrors := []error{}
 	for _, code := range pluginCode {