Don't create /nix/var/nix/{gcroots,per-user}/per-user with 1777 permission

In fact, don't create them at all because Nix does that automatically.

Also remove modules/programs/shell.nix because everything it did is
now done automatically by Nix.

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

@@ -479,21 +479,14 @@ in
 
     services.xserver.displayManager.hiddenUsers = map ({ name, ... }: name) nixbldUsers;
 
-    # FIXME: use systemd-tmpfiles to create Nix directories.
     system.activationScripts.nix = stringAfter [ "etc" "users" ]
       ''
-        # Nix initialisation.
-        install -m 0755 -d \
-          /nix/var/nix/gcroots \
-          /nix/var/nix/temproots \
-          /nix/var/nix/userpool \
-          /nix/var/nix/profiles \
-          /nix/var/nix/db \
-          /nix/var/log/nix/drvs
-        install -m 1777 -d \
-          /nix/var/nix/gcroots/per-user \
-          /nix/var/nix/profiles/per-user \
-          /nix/var/nix/gcroots/tmp
+        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
       '';
 
     nix.systemFeatures = mkDefault (