Use the python found by meson as the interpreter for installed scripts

The python interpreter found by /usr/bin/env python3 is not necessarily the same installation as the one that's found by meson's pymod.find_installation('python'). This means that even though meson is checking that the python installation it found includes the 'packaging' module, the scripts might not have access to that module when run.

For distribution packaging, it's usually desirable to have python script interpreters be fully specified paths, rather than use /usr/bin/env, to ensure the scripts run using the expected python installation (i.e. the one where the python 'packaging' dependency is installed).

The easiest way to fix this is to set the script interpreter to the full_path() of the python interpreter found by meson. The specific python interpreter that will be used can be selected through the use of a meson machine file by overriding the "python" program. Many distributions already have this set up using meson packaging helpers.

This is a potential solution to the problems brought up in #3301 (closed)

Closes #3301 (closed)