Some phases of diskimage-builder run outside the chroot environment,
such as the extra-data.d scripts, and don't have access to dib-python.
This means these scripts may choose the wrong python version by using
"#!/usr/bin/env python" to execute. The svc-map element is an example.
This patch creates a temporary directory and symbolic link for the
correct version of python, then manipulates the environment PATH
to preference the symbolic link "python" command.
This will allow elements with these scripts to work correctly with
the version of python diskimage-builder is running under.
Co-Authored-By: Ian Wienand <email@example.com>