.github/workflows/test.yml: Use Ubuntu 22.04

The packaging situation of firefox-geckodriver on Ubuntu 20.04 is a mess
and leads to broken packages:

  The following packages have unmet dependencies:
   firefox-geckodriver : Depends: firefox (= 116.0.2+build1-0ubuntu0.20.04.1) but 116.0.3+build1-0ubuntu0.20.04.1~mt1 is to be installed
  E: Unable to correct problems, you have held broken packages.

Therefore, use Ubuntu 22.04 where the firefox package includes the
selenium driver so no more version mismatch.