flatpak-run: Unset VK_DRIVER_FILES and VK_ICD_FILENAMES

These environment variables inform the Vulkan loader on where to find driver files.
Since they typically point to locations on the host filesystem, any application that
attempts to load Vulkan within the flatpak sandbox would break with these set.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f