NixOS: possibly tweak config to make it work after Spacemacs upgrade

[c0c0n3]

PR #23 upgraded my Spacemacs config. It needs testing on NixOS. I don't think there should be any issues since I've tested thoroughly the 3 config sets (editor, terminal and daemon) on MacOS with Emacs 27.2. But you never know :-)
Also I'm not using all-the-icons anymore since I've switched to Treemacs which comes with its own set of built-in icons. So
zap

• config/nixos/pkgs/fonts/emacs-all-the-icons.nix

and update NixOS expressions accordingly?