cbuild: enforce pip3 --break-system-packages when running FDO_DISTRIBUTION_EXEC
We are building system packages when we are installing those pip packages
in the containers. So there is no point in asking every users to either
use venv
(which would be counter-intuitive) or manually pass
--break-system-packages
We do that by temporary renaming the EXTERNALLY-MANAGED
file installed
by the distribution package which tells pip that it shouldn't touch
the system packages.