add Write method to config source

config/source/env/env.go

@@ -105,6 +105,10 @@ func (e *env) Watch() (source.Watcher, error) {
 	return newWatcher()
 }
 
+func (e *env) Write(cs *source.ChangeSet) error {
+	return nil
+}
+
 func (e *env) String() string {
 	return "env"
 }