Instead of hard-coding the use of the "org.osbuild.linux" runner,
use the new `osbuild.pipeline.detect_host_runner` function to
dynamically detect the runner for the host system. That should fix
the tests on RHEL systems, where python3 is by default not present
and even if it is manually installed, is an indirection via
alternatives (i.e. a link to /etc/alternatives), which must be
explicitly configured in the build root container for the host.
d7d84a8eb1