haskellPackages.ad: disable problematic test on x86_64-darwin

It is somewhat curious that it behaves differently exclusively here, but
I don't think it is necessary to stop shipping a package due to floating
point arithmetic error—it would be unreasonable to assume there were
none…

See ekmett/ad#113.

pkgs/development/haskell-modules/configuration-darwin.nix

@@ -371,4 +371,12 @@ self: super: ({
   # same
   # https://hydra.nixos.org/build/174540882/nixlog/9
   jacinda = dontCheck super.jacinda;
+
+  # Greater floating point error on x86_64-darwin (!) for some reason
+  # https://github.com/ekmett/ad/issues/113
+  ad = overrideCabal (drv: {
+    testFlags = drv.testFlags or [ ] ++ [
+      "-p" "!/issue-108/"
+    ];
+  }) super.ad;
 })