Fix Nix build

[siraben]

We might want to add a Nix CI job to ensure it doesn't get desynced. @prusnak thoughts?

[prusnak]

I prefer changing only the line mv llama $out/bin/llama to mv main $out/bin/llama and keeping the rest.

[prusnak]

I get the following error when I apply this on top of master:

> mv: cannot stat 'main': No such file or directory

It seems the binaries are now bin/main and bin/quantize ?

[siraben]

Rebuilt, it should work now.