Make the secrets commands inline aware

Rather than redirecting the global $stdout, which is not never clever in
a threaded program, we'll make the secrets commands aware they are
being inlined, so they return the value instead of printing it.

Additionally we no longer need to interrupt the parent process on error
as we've inlined the command - exit 1 is enough.

lib/kamal/secrets.rb

@@ -30,17 +30,9 @@ class Kamal::Secrets
 
     def parse_secrets
       if secrets_file
-        interrupting_parent_on_error { ::Dotenv.parse(secrets_file) }
+        ::Dotenv.parse(secrets_file)
       else
         {}
       end
     end
-
-    def interrupting_parent_on_error
-      # Make any `kamal secrets` calls in dotenv interpolation interrupt this process if there are errors
-      ENV["KAMAL_SECRETS_INT_PARENT"] = "1"
-      yield
-    ensure
-      ENV.delete("KAMAL_SECRETS_INT_PARENT")
-    end
 end