[Backport release-25.11] falcon: fix build with cmake4 - #475638

         { nixpkgs.config.allowBroken = true; }
       in configuration.nix to override this.

       c) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
         { allowBroken = true; }
       to ~/.config/nixpkgs/config.nix.

Cannot nix-instantiate `falcon.passthru.tests` because:
error: attribute 'tests' in selection path 'falcon.passthru.tests' not found