Python builder: Always use a virtualenv

The Python builder now establishes a virtualenv before performing any
other Python operations. This ensure that subsequent operations will
not affect system Python packages. This is especially important when
using a Debian Bookworm base image.

Removed support for use-system-flag, which should not be needed
anymore.

Added support for use-system-site-packages, which will pass
--system-site-packages to python -m venv.

Notes:

• Support for building wheels of requirements has been removed. The
  Python builder no longer messes with the PIP_WHEEL_DIR,
  PIP_FIND_LINKS, PIP_NO_INDEX, or PYTHONPATH environment variables.

• The prior workaround from (T346090) which set
  PIP_BREAK_SYSTEM_PACKAGES has been removed.

Bug: T357548