(from #29039)

Make the build more incremental: On installation of sage-conf (the version from pkgs/sage-conf_pypi), only run configure but do not install anything; rather, a Python entry point can be used by Python packages such as sagemath-standard to install packages if present.

Depends on #29039

Component: build

Keywords: sd111

Issue created by migration from https://trac.sagemath.org/ticket/30036