Motivation for this change

The test has recently been failing due to the IPv6 address
on the server still being in the tentative state, when the
client sends its first request. The server will not start
using the IPv6 address until DAD has completed.

Scripted networking seems not to wait for DAD completion
before completing network-online.target, so let's switch
to networkd instead, which does.

Needs a backport to 19.09, was mention in #68361.

Currently not working on master due to #70007, but other than that this makes it work again.

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 @Mic92