flat-manager-client is unreproducible due to lack of `--version` param

When using `quay.io/gnome_infrastructure/gnome-runtime-images:gnome-master` image we have access to flat-manager-client. However, we don't know which version we are using since flat-manager-client does not have something like a `--version` parameter. 

So, this makes it unreproducible since it is impossible to know on which version some bug appeared or was fixed. The same applies to features.