You must have a quicker mirror, on mine I just checked and nettle 3,6-1 is on testing core has 3.5.1-2
Soon the update xorg-server will be on obartun repositories, to start X right now you should do this:
Hopefully it is still in your cache-repository:
sudo pacman -U /var/cache/pacman/pkg/nettle-3.5.1-2-x86_64.pkg.tar.xz
If not you must download it from archive
cd /var/cache/pacman/pkg
sudo wget https://archive.archlinux.org/packages/n/nettle/nettle-3.5.1-2-x86_64.pkg.tar.xz
Then run the pacman command again
Also you may want to install this terminal because the fancier terminals may not work.
sudo pacman -S rxvt-unicode
Use it to update to nettle 3.6.
Once you are on X you have to upgrade again as some things don't work without it.