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_test.go

@@ -198,6 +198,7 @@ func TestGetPluginStatusesDisabled(t *testing.T) {
 	})
 
 	_, err := th.App.GetPluginStatuses()
+	require.NotNil(t, err)
 	require.EqualError(t, err, "GetPluginStatuses: Plugins have been disabled. Please check your logs for details., ")
 }