Upgrade MoltenVK to 1.2.8

[fornwall]

Changelog: https://github.com/KhronosGroup/MoltenVK/releases/tag/v1.2.8

Have done some basic smoke testing on wim locally.

Difference between 1.2.7 and 1.2.8 in MVK_CONFIG_LOG_LEVEL=3 cargo run --features pre-built output on a Mac M1:

-[mvk-info] MoltenVK version 1.2.7, supporting Vulkan version 1.2.275.
-	The following 108 Vulkan extensions are supported:
+[mvk-info] MoltenVK version 1.2.8, supporting Vulkan version 1.2.280.
+	The following 109 Vulkan extensions are supported:
 		VK_KHR_16bit_storage v1
 		VK_KHR_8bit_storage v1
 		VK_KHR_bind_memory2 v1
@@ -43,6 +43,7 @@
 		VK_KHR_shader_draw_parameters v1
 		VK_KHR_shader_float_controls v4
 		VK_KHR_shader_float16_int8 v1
+		VK_KHR_shader_integer_dot_product v1
 		VK_KHR_shader_non_semantic_info v1
 		VK_KHR_shader_subgroup_extended_types v1
 		VK_KHR_spirv_1_4 v1
@@ -113,7 +114,7 @@
 		type: Integrated
 		vendorID: 0x106b
 		deviceID: 0xe0303ef
-		pipelineCacheUUID: 66F6FF1E-0E03-03EF-0000-000000000000
+		pipelineCacheUUID: 1D98BABB-0E03-03EF-0000-000000000000
 		GPU memory available: 49152 MB
 		GPU memory used: 0 MB
 	supports the following Metal Versions, GPU's and Feature Sets: