Revert "Store less generations"

This reverts commit eabfb55de4.

The issues doesn't seem to be the number of generations,
but that nixos itself keeps all channels as gc roots.
This prevents proper cleanup of old channel data.

home/packages/custom/update-system/default.nix

@@ -12,7 +12,7 @@ writeShellApplication {
     nvd diff "$oldVersion" "$newVersion"
 
     home-manager expire-generations '-12 days'
-    nix-env --delete-generations +2
+    nix-env --delete-generations +3
     nix store gc
   '';
 }