This code from #29024

    os.environ["SINGULAR_BIN_DIR"] = dirname(which("Singular"))

assumes that Singular is in PATH

This causes errors when users attempt to use sagelib outside of the environment set up by sage-env.

https://groups.google.com/g/sage-support/c/XBxpl-JjaNU/m/FtRvcoW0AgAJ

Depends on #31292
Depends on #31296

CC: @orlitzky @dimpase @koffie

Component: refactoring

Author: Matthias Koeppe

Branch/Commit: 4cd3130

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/33440