nixos/build: Fix netboot initrd systemd config

`boot.initrd.systemd.extraConfig` was removed upstream and now fails
an assertion, which broke the `netbootArchive` target and with it the
second build step of the installer release workflow. Move the two
timeout settings to `settings.Manager`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

nixos/modules/build.nix

@@ -64,10 +64,10 @@ let
                 ip = "${iproute2}/bin/ip";
                 nbd-client = "${nbd}/bin/nbd-client";
               };
-              extraConfig = ''
-                DefaultTimeoutStartSec=20
-                DefaultDeviceTimeoutSec=20
-              '';
+              settings.Manager = {
+                DefaultTimeoutStartSec = "20s";
+                DefaultDeviceTimeoutSec = "20s";
+              };
 
               network = {
                 enable = true;