chore(users/sterni/emacs): use Emacs 29 instead of unstable

This means we can subtitute the base package for my emacs derivation
instead of compiling it from scratch (which is quite slow).

This means I'll be using 29.1-rc1 until
https://github.com/NixOS/nixpkgs/pull/246203 or
https://github.com/NixOS/nixpkgs/pull/247496 are merged which is okay,
as it includes pgtk support.

Change-Id: Ie7cfec1ca0df9974c049be0b00d75f32143ac905
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9065
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

users/sterni/emacs/default.nix

@@ -14,8 +14,7 @@ let
     exec chktex -n8 "$@"
   '';
 
-  # emacsPgtk is defined in emacs-overlay
-  emacs = (pkgs.emacsPackagesFor pkgs.emacs-pgtk).withPackages (epkgs: [
+  emacs = (pkgs.emacsPackagesFor pkgs.emacs29-pgtk).withPackages (epkgs: [
     epkgs.bqn-mode
     #epkgs.elpaPackages.ada-mode
     epkgs.elpaPackages.rainbow-mode