zsh: include completions for nix-* commands

(cherry picked from commit 6dbf8c04097c8961bb1cb9cb319fee42eb19ab7a)

nixos/modules/programs/zsh/zsh.nix

@@ -280,11 +280,21 @@ in
 
     environment.etc.zinputrc.source = ./zinputrc;
 
-    environment.systemPackages = [ pkgs.zsh ]
-      ++ optional
-      (cfg.enableCompletion
-        && !lib.versionAtLeast (lib.getVersion config.nix.package) "2.4pre")
-      pkgs.nix-zsh-completions;
+    environment.systemPackages =
+      let
+        completions =
+          if lib.versionAtLeast (lib.getVersion config.nix.package) "2.4pre"
+          then
+            pkgs.nix-zsh-completions.overrideAttrs
+              (_: {
+                postInstall = ''
+                  rm $out/share/zsh/site-functions/_nix
+                '';
+              })
+          else pkgs.nix-zsh-completions;
+      in
+      [ pkgs.zsh ]
+      ++ optional cfg.enableCompletion completions;
 
     environment.pathsToLink = optional cfg.enableCompletion "/share/zsh";