Update download URL for single zones when API provides it

[lanrat]

In June 2023, ICANN may change the download URL structure. For individual zones the API does not yet return this URL and uses the hard-coded URL pattern, however this may change.

https://github.com/lanrat/czds/blob/main/cmd/czds-dl/download.go#L122

From ICANN:

Additionally, users that rely on the CZDS REST API to download approved zone files should make sure to use the list of links provided by the CZDS API, as described in section 5.1 of the CZDS API specification, and be mindful that the format of the download URLs is subject to change in the near future to incorporate new query parameters. Users that may still be using a hardcoded form of the download URL as “https://czds-api.icann.org/czds/downloads/tld.zone” should update their implementation no later than June 2023.