Add back legacy .piprc customization for pip (#21124)

This change brings back backwards compatibility to using .piprc
to customize Airflow Image. Some older vrsions of pip used .piprc
(even though documentation about is difficult to find now) and we
used to support this option. With #20445, we changed to use
(fully documented) ``pip.conf`` option, however if someone used
.piprc before to customize their image, this change would break it.

The PR brings back also the .piprc option to the image (even if
it is not really clear whether current and future versions of pip
will support it.