CI uses xcb-proto from FDO

!31 (merged) added some basic CI. That CI is now failing in !49 (merged) due to

Package dependency requirement 'xcb-proto >= 1.16.0' could not be satisfied.
Package 'xcb-proto' has version '1.15.2', required version is '>= 1.16.0'

This failure exists since commit 15193346 which raises the xcb-proto dependency to "the newest one". However, our CI uses xcb-proto version "whatever FDO provides". Thus, this kind of failure will happen every time the dependency is strengthened to "something new".

CC @alanc ("you touched it last" 🙈 )