Replace Mix.env with Pleroma.Config.get(:env)

Mix.env/0 is not availible in release environments such as distillery or
elixir's built-in releases.

config/config.exs

@@ -500,6 +500,8 @@ config :pleroma, :oauth2,
 
 config :pleroma, :database, rum_enabled: false
 
+config :pleroma, :env, Mix.env()
+
 config :http_signatures,
   adapter: Pleroma.Signature