build/python: Always add the install prefix to the Guix PYTHONPATH.

This is to remove the need for common boilerplate code in check phase
overrides.

* guix/build/python-build-system.scm
(add-installed-pythonpath): Streamline.  This phase depends on the presence of
a "python" input; thus GUIX_PYTHONPATH is guaranteed to be defined.  Update doc.
(add-install-to-pythonpath): New phase.
(%standard-phases): Order it before the check phase.

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