nix flake add: Handle ~/.config/nix not existing
nix flake add: Handle ~/.config/nix not existing

Fixes

  $ nix flake add fnord github:edolstra/fnord
  error: opening file '/home/eelco/.config/nix/registry.json': No such file or directory

NixOS/nix,rycee/nix,NixOS/nix,rycee/nix,NixOS/nix,rycee/nix,NixOS/nix,NixOS/nix,rycee/nix,rycee/nix