nixos/engelsystem: pin php at 8.1

Upstream supports php 8.0/8.1 for the 3.3.0 release. The upgrade to 8.2
caused a type mismatch in carbon.

> PHP message: Exception: Code: 0, Message: Carbon\Carbon::setLastErrors(): Argument #1 ($lastErrors) must be of type array, bool given, called in /nix/store/2prnw9qya9kaks2rwvd6fkrz0c7l5ygd-engelsystem-3.3.0/share/engelsystem/vendor/nesbot/carbon/src/Carbon/Traits/Creator.php on line 98, File: vendor/nesbot/carbon/src/Carbon/Traits/Creator.php:928

nixos/modules/services/web-apps/engelsystem.nix

@@ -104,6 +104,7 @@ in {
       '';
 
     services.phpfpm.pools.engelsystem = {
+      phpPackage = pkgs.php81;
       user = "engelsystem";
       settings = {
         "listen.owner" = config.services.nginx.user;