Looks like the Arch wiki recommendation
sudo pacman -Sy archlinux-keyring
sudo pacman -Su
is generally useful to solve the libinih update issue in all cases that I have tested.
(Note that the double sudo pacman -Syu was my suggestion to Bryan, and at my first tests it seemed to fix it, but actually was not accurate. Sorry about that.)
Note also that the ISO has a separate hotfix.