Put the NixOS channel in an option

nixos/modules/misc/version.nix

@@ -24,6 +24,13 @@ with pkgs.lib;
       description = "NixOS release code name.";
     };
 
+    system.defaultChannel = mkOption {
+      internal = true;
+      type = types.uniq types.string;
+      default = https://nixos.org/channels/nixos-unstable;
+      description = "Default NixOS channel to which the root user is subscribed.";
+    };
+
   };
 
   config = {

nixos/modules/programs/shell.nix

@@ -39,7 +39,7 @@ in
 
         # Subscribe the root user to the NixOS channel by default.
         if [ "$USER" = root -a ! -e $HOME/.nix-channels ]; then
-            echo "http://nixos.org/channels/nixos-unstable nixos" > $HOME/.nix-channels
+            echo "${config.system.defaultChannel} nixos" > $HOME/.nix-channels
         fi
 
         # Create the per-user garbage collector roots directory.