glib: configure with `$PYTHON` as python

This take another approach at c5cd4e06 (component/glib: set --with-python
to generate scripts with slapos python, 2022-09-20) because this python
might exceed the limit of shebang lines, that change was bad because it
brought back the problem that we fixed in 6acdc8ca (glib: put the expected
python in $PATH, 2019-12-18)

Now that component/defaults.cfg set the ${python:location}/bin in $PATH,
we can assume that python is always in $PATH. The problem with python3
software is that python3 does not install a "python" binary, only python3,
so we can use the $PYTHON environment variable which will be set to
python2 or python3 and that in both cases this will be a valid python
interpreter.

This re-applies 7581b668 (component/glib: remove bad file bin/gtester-report
after compilation, 2020-08-07) because with this approach that script's
shebang is just #!python3