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/default.nix

@@ -35,7 +35,7 @@
 
     packages = with pkgs; [
       impala
-      inputs.yogurt.packages.${pkgs.system}.default
+      inputs.yogurt.packages.${pkgs.stdenv.hostPlatform.system}.default
     ];
 
     username = lib.mkDefault "naps62";