Reset the env before pushing

Calling `load_envs` again does not load updated env variables, because
Dotenv does not overwrite existing values.

To fix this we'll store the original ENV and reset to it before
reloading.

https://github.com/basecamp/kamal/issues/512

lib/kamal/cli/base.rb

@@ -24,6 +24,7 @@ module Kamal::Cli
 
     def initialize(*)
       super
+      @original_env = ENV.to_h.dup
       load_envs
       initialize_commander(options_with_subcommand_class_options)
     end
@@ -37,6 +38,12 @@ module Kamal::Cli
         end
       end
 
+      def reload_envs
+        ENV.clear
+        ENV.update(@original_env)
+        load_envs
+      end
+
       def options_with_subcommand_class_options
         options.merge(@_initializer.last[:class_options] || {})
       end