zsh: Patch hardcoded histsize out of grml’s config

pkgs/default.nix

@@ -20,6 +20,14 @@ in
     withBDplus = true;
   };
 
+  grml-zsh-config = prev.grml-zsh-config.overrideAttrs (o: o // {
+    postPatch = (o.postPatch or "") + ''
+      substituteInPlace etc/zsh/zshrc \
+          --replace 'isgrmlcd && HISTSIZE=500  || HISTSIZE=5000' "" \
+          --replace 'isgrmlcd && SAVEHIST=1000 || SAVEHIST=10000' ""
+    '';
+  });
+
   # adapted from https://github.com/NixOS/nixpkgs/pull/85790
   # TODO: Remove when memtest86+ ≥ 5.31b is in nixpkgs
   memtest86plus = prev.memtest86plus.overrideAttrs (o: o // rec {