nixos/systemd-udev-settle: don't restart on upgrades

The idea is that we only need this target during boot,
however there is no point on restarting it on every upgrade.

This hopefully fixes NixOS#21954

nixos/modules/system/boot/systemd.nix

@@ -898,6 +898,7 @@ in
     systemd.services.systemd-remount-fs.restartIfChanged = false;
     systemd.services.systemd-update-utmp.restartIfChanged = false;
     systemd.services.systemd-user-sessions.restartIfChanged = false; # Restart kills all active sessions.
+    systemd.services.systemd-udev-settle.restartIfChanged = false; # Causes long delays in nixos-rebuild
     # Restarting systemd-logind breaks X11
     # - upstream commit: https://cgit.freedesktop.org/xorg/xserver/commit/?id=dc48bd653c7e101
     # - systemd announcement: https://github.com/systemd/systemd/blob/22043e4317ecd2bc7834b48a6d364de76bb26d91/NEWS#L103-L112