protobuf: downgrade default to 3.21 on darwin

Feel free to find a better solution. This is just a simple
mass-regression stop-gap, as noone seems to care about the failure.

pkgs/top-level/all-packages.nix

···
 
   prospector = callPackage ../development/tools/prospector { };
 
-  protobuf = protobuf3_24;
+  # 3.24 and 3.23 tests crash on Hydra for *-darwin:
+  # https://hydra.nixos.org/build/235677717/nixlog/4/tail
+  protobuf = if stdenv.isDarwin then protobuf3_21 else protobuf3_24;
 
   protobuf3_24 = callPackage ../development/libraries/protobuf/3.24.nix { };
   protobuf3_23 = callPackage ../development/libraries/protobuf/3.23.nix { };