nixos/config/nix-channel: Factor out root channel initialization

nixos/modules/config/nix-channel.nix

@@ -0,0 +1,35 @@
+{ config, lib, ... }:
+let
+  inherit (lib)
+    mkIf
+    mkOption
+    stringAfter
+    types
+    ;
+
+  cfg = config.nix;
+
+in
+{
+  options = {
+    system = {
+      defaultChannel = mkOption {
+        internal = true;
+        type = types.str;
+        default = "https://nixos.org/channels/nixos-unstable";
+        description = lib.mdDoc "Default NixOS channel to which the root user is subscribed.";
+      };
+    };
+  };
+
+  config = mkIf cfg.enable {
+
+    system.activationScripts.nix-channel = stringAfter [ "etc" "users" ]
+      ''
+        # Subscribe the root user to the NixOS channel by default.
+        if [ ! -e "/root/.nix-channels" ]; then
+            echo "${config.system.defaultChannel} nixos" > "/root/.nix-channels"
+        fi
+      '';
+  };
+}

nixos/modules/misc/version.nix

@@ -140,13 +140,6 @@ in
       '';
     };
 
-    defaultChannel = mkOption {
-      internal = true;
-      type = types.str;
-      default = "https://nixos.org/channels/nixos-unstable";
-      description = lib.mdDoc "Default NixOS channel to which the root user is subscribed.";
-    };
-
     configurationRevision = mkOption {
       type = types.nullOr types.str;
       default = null;

nixos/modules/module-list.nix

@@ -18,6 +18,7 @@
   ./config/mysql.nix
   ./config/networking.nix
   ./config/nix.nix
+  ./config/nix-channel.nix
   ./config/nix-remote-build.nix
   ./config/no-x-libs.nix
   ./config/nsswitch.nix

nixos/modules/services/misc/nix-daemon.nix

@@ -263,11 +263,6 @@ in
     system.activationScripts.nix = stringAfter [ "etc" "users" ]
       ''
         install -m 0755 -d /nix/var/nix/{gcroots,profiles}/per-user
-
-        # Subscribe the root user to the NixOS channel by default.
-        if [ ! -e "/root/.nix-channels" ]; then
-            echo "${config.system.defaultChannel} nixos" > "/root/.nix-channels"
-        fi
       '';
 
     # Legacy configuration conversion.