perf(disko): use nix-shell instead of nix run
See https://github.com/nix-community/disko/issues/947#issuecomment-2685647104
> Issue 1
> - disko-install downloads everything to /nix/store, but nixos-install - to /mnt/nix/store directly
> Issue 2:
> - nix run 'github:nix-community/disko/latest#disko-install' takes 1.3G
> - nix-shell -p disko just took 1M