chore: emacs28 -> emacs(29)

Use the default emacs attribute over the versioned emacs28 attribute. On
current canon, this implies an upgrade to Emacs 29.

Emacs < 30 has some CVEs that have never been patched in those release
series. The next channel bump will upgrade pkgs.emacs to Emacs 30, so
just using the default alias seems to be the easiest solution.

See also <https://github.com/NixOS/nixpkgs/pull/386174>.

Change-Id: I1580bae138cc0801e0f3431b1a02c8a4585c2996
Reviewed-on: https://cl.tvl.fyi/c/depot/+/13216
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

nix/writeElispBin/default.nix

@@ -1,6 +1,6 @@
 { depot, pkgs, ... }:
 
-{ name, src, deps ? (_: [ ]), emacs ? pkgs.emacs28-nox }:
+{ name, src, deps ? (_: [ ]), emacs ? pkgs.emacs-nox }:
 
 let
   inherit (pkgs) emacsPackages emacsPackagesFor;