vulkan-loader: Always include /run/opengl-driver(-32)/share in search path.

Even though FALLBACK_DATA_DIRS is set to include this, it only applies when XDG_DATA_DIRS is not defined, so the NixOS opengl.nix module still had to include these in the search path. Use a simple patch to force a default search path, consulted after all other search paths.

Note that FALLBACK_DATA_DIRS is no longer set, and the default (/usr/local/share:/usr/share) applies.