Environment variable file is not named correctly on Windows

When RabbitMQ environment variable file is generated, we [assume](https://github.com/rabbitmq/chef-cookbook/blob/8946d0bd6096392ea62b9a7e08b6fa749072d639/recipes/default.rb#L314) that it is named `rabbitmq-env.conf` on all platforms but this is not the case on Windows, where the name is `rabbitmq-env-conf.bat`.