k3s.tests: fix eval

nixos/tests/k3s/default.nix

···
     n: _: lib.strings.hasPrefix "k3s_" n && (builtins.tryEval pkgs.${n}).success
   ) pkgs;
 in
-lib.recurseIntoAttrs {
+{
   airgap-images = lib.mapAttrs (
     _: k3s: import ./airgap-images.nix { inherit system pkgs k3s; }
   ) allK3s;