scripts: Improve paths in update_ext_resources.sh
scripts: Improve paths in update_ext_resources.sh

Script was relying on $PWD for determining the current path which
is unreliable in some situations. Updated to a more robust method
allowing spaces in paths.

Change-Id: Ibdb780c479ad7fa831d113e445cc598830806806

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers