Use env files for secrets

Add env files back in for secrets - hides them from process lists and
allows you to pick up the latest env file when running
`kamal app exec` without reusing.

lib/kamal/configuration.rb

@@ -217,7 +217,7 @@ class Kamal::Configuration
   end
 
 
-  def host_env_directory
+  def env_directory
     File.join(run_directory, "env")
   end