nixos/nix-channel: default to channels.nixos.org

Infra is interested in deprecating the redirect over the domain apex
eventually, since it is just a redirect to channels.nixos.org, which
we can more reliably redistribute with a CDN.

The nixos.org domain currently offers no IPv6 due to issues we've had
with our DNS hoster. (cf. NixOS/infra#873)

Author: mweinelt

nixos/modules/config/nix-channel.nix

@@ -71,7 +71,7 @@ in
       defaultChannel = mkOption {
         internal = true;
         type = types.str;
-        default = "https://nixos.org/channels/nixos-unstable";
+        default = "https://channels.nixos.org/nixos-unstable";
         description = "Default NixOS channel to which the root user is subscribed.";
       };
     };