The ubuntu-20.04 machine has php 7.4 and 8.0 installed.
configure.ac always looks for newer versions over older versions of php.
To prevent always testing 8.0, remove all versions and just install the
required version.
It is not available out of the box in GitHub environment, unlike under
Travis CI, so install it ourselves.
Note that we do it manually because installing the rael-gc/rvm PPA
doesn't seem to work with weird failures in GitHub Actions environment.
We also can't use gpg2 --recv-keys in this environment as it doesn't
work, apparently due to a firewall.
