nixos/engelsystem: don't rely on `mysql` alias

Since 3edde6562e, we can no longer use
aliases inside the test framework. This has the implication that we can
no longer use aliases in any NixOS modules used by the test framework as
well (which is good), but does mean we need to clean up any instances
where this is the case.

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

@@ -89,7 +89,7 @@ in {
     # create database
     services.mysql = mkIf cfg.createDatabase {
       enable = true;
-      package = mkDefault pkgs.mysql;
+      package = mkDefault pkgs.mariadb;
       ensureUsers = [{
         name = "engelsystem";
         ensurePermissions = { "engelsystem.*" = "ALL PRIVILEGES"; };