Mk/Uses/python.mk: Add USE_PYTHON=build to support pyproject.toml based projects

USE_PYTHON=build supports PEP-517 at the level of individual ports.
Global support (making PEP-517 be used for all ports) is outside of the
scope of this patch.

PR: 255722
Approved by: python (maintainer's timeout; 14 days)
Differential Revision: https://reviews.freebsd.org/D36061