Building python-gst fails with:
Unbound variable: python-version
Therefore, add python-build-system to #:modules so that it's imported, and
give it a prefix to avoid clashing with the gnu-build-system.
Also, simplify the code by using the site-packages function from the
python-build-system module.
* gnu/packages/gstreamer.scm (python-gst)[arguments]: Rework to fix build
failure.