Instruct NixOS users to build their own kernel

The NixOS instructions don't work for now, at least for Intel Surface Pro.

It seems that nixos-hardware has made an oversight so that they're pulling a commit of linux-surface that doesn't have the 6.0 kernel anymore, and then trying to build the 6.0 kernel. Background: https://discourse.nixos.org/t/surface-pro-4-nixos-hardware-nixos-rebuild-error/27101 There are also some other issues with IPTSD.

I propose adding an alternative instruction to the install page, that NixOS users can just specify their own kernel build, linking to the example NixOS/nixpkgs#240612 (comment)