Download `Slang` only when `LVK_WITH_SLANG` is enabled #533

Job	Run time
Android (Ubuntu)	4m 45s
cmake-build-macos (macOS - Clang (Xcode), macos-latest, Debug, clang, clang++, Xcode, -DLVK_WITH_...	3m 0s
cmake-build (Windows - MSVC 2022 (glslang), windows-latest, Debug, cl, cl, Visual Studio 17 2022,...	2m 53s
cmake-build (Windows - MSVC 2022 (Slang), windows-latest, Debug, cl, cl, Visual Studio 17 2022, -...	8m 15s
cmake-build (Ubuntu - Clang, ubuntu-latest, Debug, clang-15, clang++-15, Unix Makefiles)	3m 35s
cmake-build (Ubuntu - Clang (Wayland), ubuntu-latest, Debug, clang-15, clang++-15, Unix Makefiles...	3m 24s
	25m 52s

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