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.

lib/pleroma/plugs/http_security_plug.ex

@@ -56,14 +56,14 @@ defmodule Pleroma.Plugs.HTTPSecurityPlug do
     connect_src = "connect-src 'self' #{static_url} #{websocket_url}"
 
     connect_src =
-      if Mix.env() == :dev do
+      if Pleroma.Config.get(:env) == :dev do
         connect_src <> " http://localhost:3035/"
       else
         connect_src
       end
 
     script_src =
-      if Mix.env() == :dev do
+      if Pleroma.Config.get(:env) == :dev do
         "script-src 'self' 'unsafe-eval'"
       else
         "script-src 'self'"