Non-deterministic cook due to UModel::LightingGuid

The -DiffOnly cook mode shows that the LightingGuid seems to change every cook. In Lyra, there seems to be 48 such assets that change every cook due to this.

1. Cook Lyra for Windows
2. Cook Lyra for Windows again with the command line argument -DiffOnly
3. Observe LogArchiveDiff warnings in the log that contain "Serialized Object: Model"

UnrealEditor-Core.dll!operator<<() [E:\Release\Engine\Source\Runtime\Core\Private\Misc\Guid.cpp:505]
UnrealEditor-Engine.dll!UModel::Serialize() [E:\Release\Engine\Source\Runtime\Engine\Private\Model.cpp:315]
UnrealEditor-CoreUObject.dll!`anonymous namespace'::WriteExports() [E:\Release\Engine\Source\Runtime\CoreUObject\Private\UObject\SavePackage2.cpp:1870]
UnrealEditor-CoreUObject.dll!`anonymous namespace'::SaveHarvestedRealms() [E:\Release\Engine\Source\Runtime\CoreUObject\Private\UObject\SavePackage2.cpp:2383]
UnrealEditor-CoreUObject.dll!`anonymous namespace'::InnerSave() [E:\Release\Engine\Source\Runtime\CoreUObject\Private\UObject\SavePackage2.cpp:2539]
UnrealEditor-CoreUObject.dll!UPackage::Save2() [E:\Release\Engine\Source\Runtime\CoreUObject\Private\UObject\SavePackage2.cpp:2696]
UnrealEditor-CoreUObject.dll!UPackage::Save() [E:\Release\Engine\Source\Runtime\CoreUObject\Private\UObject\SavePackage.cpp:39]