This is part of #21707 - "Split sage-env into 5".

We set up a new configure-generated, to-be-sourced shell script build/bin/sage-build-env-config to which we move environment variable settings from src/bin/sage-env[-config] that are only needed for:

1. Sage-the-distribution while building spkgs,
2. Sage-the-distribution for building sagelib.

The new script is used in addition to src/bin/sage-env in these two contexts.

We add a command sage --buildsh, like sage --sh but using the larger environment.

Follow-up:

• build/make/install contains more environment settings that should be reviewed for moving to sage-build-env-config

Depends on #29233

CC: @dimpase @embray @jdemeyer @kiwifb @isuruf

Component: build

Author: Matthias Koeppe

Branch/Commit: 2ab9b09

Reviewer: Dima Pasechnik, Volker Braun

Issue created by migration from https://trac.sagemath.org/ticket/29052