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/module-list.nix

@@ -142,7 +142,6 @@
   ./programs/seahorse.nix
   ./programs/slock.nix
   ./programs/shadow.nix
-  ./programs/shell.nix
   ./programs/spacefm.nix
   ./programs/singularity.nix
   ./programs/ssh.nix

nixos/modules/programs/shell.nix

@@ -1,54 +0,0 @@
-# This module defines a standard configuration for NixOS shells.
-
-{ config, lib, ... }:
-
-with lib;
-
-{
-
-  config = {
-
-    environment.shellInit =
-      ''
-        # Set up the per-user profile.
-        mkdir -m 0755 -p "$NIX_USER_PROFILE_DIR"
-        if [ "$(stat -c '%u' "$NIX_USER_PROFILE_DIR")" != "$(id -u)" ]; then
-            echo "WARNING: the per-user profile dir $NIX_USER_PROFILE_DIR should belong to user id $(id -u)" >&2
-        fi
-
-        if [ -w "$HOME" ]; then
-          if ! [ -L "$HOME/.nix-profile" ]; then
-              if [ "$USER" != root ]; then
-                  ln -s "$NIX_USER_PROFILE_DIR/profile" "$HOME/.nix-profile"
-              else
-                  # Root installs in the system-wide profile by default.
-                  ln -s /nix/var/nix/profiles/default "$HOME/.nix-profile"
-              fi
-          fi
-
-          # Subscribe the root user to the NixOS channel by default.
-          if [ "$USER" = root -a ! -e "$HOME/.nix-channels" ]; then
-              echo "${config.system.defaultChannel} nixos" > "$HOME/.nix-channels"
-          fi
-
-          # Create the per-user garbage collector roots directory.
-          NIX_USER_GCROOTS_DIR="/nix/var/nix/gcroots/per-user/$USER"
-          mkdir -m 0755 -p "$NIX_USER_GCROOTS_DIR"
-          if [ "$(stat -c '%u' "$NIX_USER_GCROOTS_DIR")" != "$(id -u)" ]; then
-              echo "WARNING: the per-user gcroots dir $NIX_USER_GCROOTS_DIR should belong to user id $(id -u)" >&2
-          fi
-
-          # Set up a default Nix expression from which to install stuff.
-          if [ ! -e "$HOME/.nix-defexpr" -o -L "$HOME/.nix-defexpr" ]; then
-              rm -f "$HOME/.nix-defexpr"
-              mkdir -p "$HOME/.nix-defexpr"
-              if [ "$USER" != root ]; then
-                  ln -s /nix/var/nix/profiles/per-user/root/channels "$HOME/.nix-defexpr/channels_root"
-              fi
-          fi
-        fi
-      '';
-
-  };
-
-}

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 (