nixos/networking: make /etc/netgroup by default

[omasanori]

Motivation for this change

I had removed the PR branch for #102870 by mistake so the PR was closed and unable to be reopened. Thus I restart a PR here. This is my fault, sorry.

NixOS now generates an empty file /etc/netgroup by default. It will suppress the "/etc/netgroup: No such file or directory" messages from nscd.
Inspired by #68671 but I changed config/networking.nix instead of services/networking/nscd.nix since the file is not specific to glibc.

Fixes #68603

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• 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 nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[flokli]

Thanks!