agdaPackages.iowa-stdlib: don't try to build on Hydra

It's marked as broken which always creates a lot of clutter in hydra
evaluation error pages, so we'll just set hydraPlatforms = [] for now.

pkgs/development/libraries/agda/iowa-stdlib/default.nix

@@ -26,6 +26,8 @@ mkDerivation (rec {
     platforms = lib.platforms.unix;
     # broken since Agda 2.6.1
     broken = true;
+    # remove once unbroken
+    hydraPlatforms = lib.platforms.none;
     maintainers = with lib.maintainers; [ alexarice turion ];
   };
 })