tabby: support vulkan backend and consider defaulting to it

[LunNova]

Using tabby is complicated by needing a different configuration depending on the hardware and versions of kernel modules in use on the host machine.

tabby recently gained support for a vulkan backend which cares much less about having specific versions of nvidia or AMD kernel modules and supports both vendors with one build.

Making the vulkan backend the default once it's supported would be neat!

I tried it out with this modified tabby/default.nix and it seems to work but I made a mess of the cuda/rocm support in the process.

cc @ghthor