Disable Hydra CI for darwin runners

The IOG Hydra CI is often failing on the darwin runners and we want to
have less false negatives on our tests. Now that we have own github
actions, this should only impact nix cache hits on cache.iog.io

flake.nix

@@ -63,7 +63,7 @@
         };
 
         # Build selected derivations in CI for caching
-        hydraJobs = {
+        hydraJobs = pkgs.lib.optionalAttrs (!pkgs.stdenv.isDarwin) {
           packages = { inherit (packages) hydra-node hydra-tui hydraw spec; };
           devShells = { inherit (devShells) default ci; };
         };