p11-kit: only cause rebuild on x86_64-darwin where it fails

Fixes: 34f5abab7a
(cherry picked from commit 2cb56f387a)

pkgs/development/libraries/p11-kit/default.nix

@@ -67,7 +67,7 @@ stdenv.mkDerivation rec {
     ))
   ];
 
-  mesonCheckFlags = [
+  ${if stdenv.buildPlatform.isDarwin && stdenv.buildPlatform.isx86_64 then "mesonCheckFlags" else null} = [
     # Tests regularly exceed the default timeout on `x86_64-darwin`.
     "--timeout-multiplier=0"
   ];