Fix notifications if env is a symbol

I'm not quite sure why and since when but our deploy notifications weren't working anymore. I tracked it down to the `env` that's passed in from Capistrano being a symbol instead of a string so `load_config_from_disk` couldn't find the correct configuration.

We use `capistrano/ext/multistage` other than that it's just vanilla Capistrano usage. The weird thing is that this used to work without problems and we haven't changed anything. I couldn't really find any relevant commits in Appsignal either.