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/elixir.nix

@@ -1,6 +1,7 @@
 { pkgs, ... }:
 {
   home.packages = with pkgs; [
-    elixir_1_18
+    # top-level elixir_* aliases are deprecated in favour of the beamPackages sets
+    beamPackages.elixir_1_18
   ];
 }