While switching NixOS configurations with both

networking.useNetworkd = true;
virtualisation.virtualbox.host.enable;

You often end up waiting for systemd-networkd-wait-online.service.

This happens because the vboxnet0 device doesn't have a carrier until
virtualbox machines are started, so networkd gets stuck in
"Configuring":

⇒  networkctl list
IDX LINK          TYPE      OPERATIONAL SETUP
  1 lo            loopback  carrier     unmanaged
  2 wlp2s0        wlan      routable    unmanaged
  3 vboxnet0      ether     no-carrier  configuring

This updates the NixOS virtualbox host module to include a
RequiredForOnline=no statement in the generated 40-vboxnet0.network
file, so networkd doesn't consider it necessary for
systemd-networkd-wait-online.service to finish.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• 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 nix-review --run "nix-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.

Notify maintainers

cc @