Stale references to nix + other issues

Here are a couple of issues with the nix component of this guide:

1. The Build an air-gapped NixOS LiveCD image part of the guide no longer works after moving flake.nix to nix subdirectory. Similar issue with the other commands. Adding ?dir=nix argument to the flake url should solve the problem.
2. The flake lock file is outdated: it contains drduhConfig which was removed from the flake inputs.

And other issues noticed while following it:

1. sudo mkdir /mnt/encrypted-storage does not work on NixOS as /mnt is not created by default.
2. gpg-agent needs to be stopped before using ykman openpgp commands.
3. Transfer subkeys just doesn't work (gpg: KEYTOCARD failed: Invalid time error). What solved was to run the interactive command manually without --pinentry-mode=loopback
4. Running save after keytocard makes it annoying to transfer the keys to multiple yubikeys, as they are removed from gnupg. I had to delete all secret keys form my gnupg and re-import the backups. It seems to me like the live NixOS image support is slowly being faded out. Is there a particular reason for this?

Anyways, this was a great guide, thank you to all who contributed to it!