The root cause is the upgrade of json-schema to v6 because of jsonrainbow/json-schema#816.

shivammatur/setup-php supports a way of using a 32bits build of PHP in CI on Ubuntu. See https://github.com/shivammathur/setup-php#multi-arch-setup and shivammathur/setup-php#302 (comment)

It could make sense to add a job running on 32bits to ensure Composer stays compatible with such runtime.