JIRA INFRA-6368

with Debian based operating systems.

When installing conda with the installer, conda can be updated by using
conda. But when using the RPM, conda should only be updated with yum.

The RPMs provided by conda are currently updated very seldomly (conda
4.6.11 is available, but the only rpm available is still 4.5.12).
Using the installer gives better control of the version we want to
install.