add swaylock to env in swaylock module

this mirrors gtklock

hosts/common/optional/swaylock.nix

@@ -1,5 +1,6 @@
 {
     config,
+    pkgs,
     ...
 }: let
     inherit (config.poz.other.system) username;
@@ -7,6 +8,8 @@ in {
     # https://github.com/NixOS/nixpkgs/issues/158025
     security.pam.services.swaylock.text = "auth include login";
 
+    users.users.${username}.packages = [ pkgs.swaylock ];
+
     home-manager.users.${username} = {
         programs.swaylock = {
             enable = true;