runners: use platform-python on RHEL

Runner are invoked to prepare the execution of stages and assemblers
inside the container. The setup tasks are specific to the distribution
and maybe the version of it, therefore specific runners are used for
each distribution+version combination.
The build the first (most nested) build root, `/usr` is taken from the
host to bootstrap the container. On RHEL, the python interpreter to be
used for software that belongs to the platform is platform-python, as
it provides a stable API. Therefore the RHEL runners should use that
instead of relying on the presence of /usr/bin/python3.6, which might
not be installed and is indeed not installed by default.

runners/org.osbuild.rhel81

@@ -1,4 +1,4 @@
-#!/usr/bin/python3.6
+#!/usr/libexec/platform-python
 
 import os
 import subprocess

runners/org.osbuild.rhel82

@@ -1,4 +1,4 @@
-#!/usr/bin/python3.6
+#!/usr/libexec/platform-python
 
 import os
 import subprocess