Fix buildroot problems with `/usr/bin/python3` by adding a highest-priority `alternatives` for this path to the python36 buildroot-only package.

`/usr/bin/python3` in RHEL 8 is usser-configurable through `alternatives`. The definitions for these alternatives are defined in each Python alternative stack: python36, python38, python39, python3.11, etc. These definitions were never meant to be added to buildroot, but in the end they were. We need to fix the python36 buildroot-only package by providing an alternative to `/usr/bin/python3` pointing to Python 3.6 (platform-python), and make the alternative the highest priority, so it becomes default.

Process note
Since this is a buildroot-only package, Errata should not be filed (and if automation creates it, it should be dropped). See process for buildroot only packages:
https://one.redhat.com/rhel-development-guide/#proc_updating-a-buildroot-only-package