lib.systems: add darwinPlatform where necessary

lib/systems/examples.nix

···
     sdkVer = "14.3";
     xcodeVer = "12.3";
     xcodePlatform = "iPhoneSimulator";
+    darwinPlatform = "ios-simulator";
     useiOSPrebuilt = true;
   };
 
···
     sdkVer = "14.3";
     xcodeVer = "12.3";
     xcodePlatform = "iPhoneSimulator";
+    darwinPlatform = "ios-simulator";
     useiOSPrebuilt = true;
   };