nixos/tests/predictable-interface-names: Fix systemd reference

pkgs.systemd does not respect config.systemd.package and
config.boot.initrd.systemd.package overrides. Fix references to the
systemd package to ensure that the udevadm used is copied into the
initrd.

(cherry picked from commit 60be170a3a)

nixos/tests/predictable-interface-names.nix

@@ -43,10 +43,15 @@ pkgs.lib.listToAttrs (
         meta = { };
 
         nodes.machine =
-          { pkgs, lib, ... }:
+          {
+            config,
+            pkgs,
+            lib,
+            ...
+          }:
           let
             script = ''
-              ${lib.getExe' pkgs.systemd "udevadm"} settle --timeout=180
+              ${lib.getExe' config.boot.initrd.systemd.package "udevadm"} settle --timeout=180
               ip link
               if ${lib.optionalString predictable "!"} ip link show eth0; then
                 echo Success