[pretalx] pin package to unstable to fix security problem

machines/pretalx/configuration.nix

@@ -1,4 +1,4 @@
-{ config, self, lib, pkgs,  ... }:
+{ config, self, lib, pkgs, inputs,  ... }:
 
 with lib;
 
@@ -58,6 +58,7 @@ in
 
   services.pretalx = {
     enable = true;
+    package = inputs.nixpkgs-unstable.legacyPackages."x86_64-linux".pretalx;
     celery.extraArgs = [
       "--concurrency=${toString config.microvm.vcpu}"
     ];