The mistake made in #30912 broke the direct use of src/setup.py. (Did not affect normal build.)

Depends on #30912

CC: @jhpalmieri @tobiasdiez @isuruf

Component: build

Author: Matthias Koeppe

Branch/Commit: 043d3ab

Reviewer: John Palmieri

Issue created by migration from https://trac.sagemath.org/ticket/31357