gh-38752: build/pkgs/sagetex/dependencies: add $(PYTHON_TOOLCHAIN)
Our sagetex package needs python_build, included as part of
`$(PYTHON_TOOLCHAIN)`, to build.
Closes: https://github.com/sagemath/sage/issues/38657
URL: https://github.com/sagemath/sage/pull/38752
Reported by: Michael Orlitzky
Reviewer(s): Matthias Köppe
