home: clear four eval warnings

All pre-existing, all mechanical:

- pointerCursor no longer implies generation from the block alone; set enable.
- top-level elixir_1_18 alias is deprecated for the beamPackages set. Same
  derivation, 1.18.4 either way.
- pkgs.system -> pkgs.stdenv.hostPlatform.system.
- fzf and atuin both bind Ctrl-R. Atuin is sourced last so it already owned the
  key; historyWidget.zsh.command = "" states that explicitly and satisfies
  home-manager's conflict check. Behaviour is unchanged and fzf keeps Ctrl-T
  and Alt-C.

home/common/programs/desktop/default.nix

@@ -57,6 +57,8 @@
     ];
 
     pointerCursor = {
+      # setting the block alone no longer implies generation; must be explicit
+      enable = true;
       package = pkgs.numix-cursor-theme;
       name = "Numix-Cursor-Light";
     };