vulkan: use VK_SDK_PATH env var in meson
To detect the SDK and especially a greater version of the vulkan headers, meson can define the vulkan_inc_dir to a path set by VK_SDK_PATH env var. It keeps consistency with Windows Vulkan detection.
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To detect the SDK and especially a greater version of the vulkan headers, meson can define the vulkan_inc_dir to a path set by VK_SDK_PATH env var. It keeps consistency with Windows Vulkan detection.