Clean-up for src/bin/sage-env, move src/bin/sage-clone-source, src/bin/sage-sdist to build/bin #29825
Description
We remove some unused environment variables from the sage runtime environment.
Part of Meta-ticket #21707: Split sage-env into 5 to clean up sage configuration
Depends on #21559
Depends on #29929
Depends on #30088
CC: @orlitzky @kiwifb @antonio-rojas @vbraun
Component: build
Author: Matthias Koeppe
Branch/Commit: 7c91fb4
Reviewer: John Palmieri, François Bissey
Issue created by migration from https://trac.sagemath.org/ticket/29825