nixos/dhcpcd: Wait for devices to settle, unblock nixos-unstable

[srhb]

Motivation for this change

This works around #45346 and removes the channel blocker on unstable by waiting for the devices to settle via udevadm. This is needed to avoid a race condition where dhcpcd grabs the interface before predictable interface renaming is done after dhcpcd was moved to before networking.target in #44524

I still feel that it seems more logical that dhcpcd should be after networking.target (all interfaces are ready and renamed,) but since that is not desired (per the discussion in the aforementioned issue,) this should work.

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 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.

---

[globin]

Also see #39329 which was opened when we hat this issue with networkd.

Also maybe after = [ "systemd-udev-settle.service" ];?

[srhb]

@globin Ah, yes, that makes sense. Thanks. :)