gnome.nixos-gsettings-overrides: Ensure the settings are not overwritten

https://github.com/NixOS/nixpkgs/commit/1d4bddaed58c23fbfb4df0bf80d5ffd6803a6a34
started copying override files from packages into the schema for compilation.
`gsettings-desktop-schemas` package contains `remove-backgrounds.gschema.override`
which is later in the alphabet than `nixos-defaults.gschema.override`
so it would take precedence over the values from the GNOME NixOS module,
causing the background to not be set, among other things.

Let’s rename the override file coming from the module to be applied last.

pkgs/desktops/gnome/nixos/gsettings-overrides/default.nix

···
   ${concatMapStringsSep "\n" (pkg: "cp -rf \"${glib.getSchemaPath pkg}\"/*.xml \"${glib.getSchemaPath pkg}\"/*.gschema.override \"$schema_dir\"") gsettingsOverridePackages}
 
   chmod -R a+w "$data_dir"
-  cat - > "$schema_dir/nixos-defaults.gschema.override" <<- EOF
+  cat - > "$schema_dir/zz-nixos-defaults.gschema.override" <<- EOF
   ${gsettingsOverrides}
   EOF