previous mkDefault did not work as expected,
as it did not overwrite the original submodule's defaults when the user
did not specify any custom options at all.

Motivation for this change

Resolves issue brought up at #28749 (comment) . recursiveUpdate was what I needed! With this, the nginx option of piwik will now behave exactly the same like before the nginx virtualHost submodule was used.

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.