nix: move krb5 options to security section

With an update to nix, they moved where you have to specify krb5 settings… hopefully this is all that needs changed. See these links:

1. https://github.com/NixOS/nixpkgs/commit/92a541c0ed590db1e8bee7436a6130cb5d589a6c
2. https://github.com/NixOS/nixpkgs/commit/fed77d170513ea7a09c8ed5ec5d3eaa8fdd0fd87#diff-9c253a17d297f3da51e6ef73a3cb898d879ccf8b0c8613199770917e9387d92c
3. https://nixos.org/manual/nixos/unstable/release-notes.html