Fix split email in self module

[darkone-linux]

This fix addresses two issues:

• The splitString "." on the email could incorrectly capture the @.
  john.doe@domain.tld -> john, doe@domain
• The argument order for elemAt was most likely reversed.

To test quickly:

nix-instantiate --eval --strict -E '
let
  pkgs = import (import ./npins).nixpkgs {};
  inherit (pkgs) lib;
  selfModule = import ./modules/self.nix;
  eval = lib.evalModules {
    modules = [
      selfModule
      {
        _module.args.vpnProfiles = {};
        _module.check = false;
      }
      {
        securix.self = {
          mainDisk = "/dev/nvme0n1";
          edition = "test";
          user.email = "prenom.nom@domaine.tld";
          machine = {
            hardwareSKU = "x280";
            serialNumber = "000000";
          };
        };
      }
    ];
  };
in {
  username = eval.config.securix.self.user.username;
}
'

It should normally produce:

{ username = "pnom"; }

NB: It might make sense to move this kind of processing into a dedicated library, using testable helper functions. That would make the algorithm more robust and allow handling additional cases, for example email addresses that do not follow the "firstname.lastname..." pattern.

[jdauphant-dinum]

deriveUsernameFromEmail is not used, we will remove the code