The environment variable DISTRO is used to set the target Docker image
used when running the Mitogen unit tests. DISTROS is used for the
Ansible integration tests. VER sets the version of Ansible that is
installed on the controller.
When MODE=Mitogen
- there is no need to set VER, because nothing that installs Ansible
- it does not make sense to set DISTROS.
Unlike on Debian, some environment variables that tickle
getpass.getuser() are being inherited. So use getuid() instead.
Also install the doas binary on CentOS. CI was changed (I believe) to
shrink the configuration matrix, and now these tests run on CentOS too.
Alex Willmer