Motivation for this change

Some modules of cloud-init can cope with a network not immediately
available (notably, the EC2 module), but some others won't retry if
network is not available (notably, the Cloudstack module).
network.target doesn't give much guarantee about the network
availability. Applications not able to start without a fully
configured network should be ordered after network-online.target.

Also see #44573 and #44524.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS (booted on a Cloudstack platform)
  • 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 nox --run "nox-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)
• Fits CONTRIBUTING.md.