Follow-up from #29038.

We clean up sage-env-config, intended for setting direct values only, by moving conditional settings of other environment variables to sage-env instead.

Moreover, we move configuration variables that are only needed by the Sage python process but not as environment variables by its child processes, to sage_conf.py. In this way they become available in the sage.all module even when invoked outside of a sage-env.

(part of #21707.)

CC: @orlitzky @dimpase @jhpalmieri

Component: build: configure

Author: Matthias Koeppe

Branch/Commit: 7364e2a

Reviewer: Michael Orlitzky

Issue created by migration from https://trac.sagemath.org/ticket/29384