Make sure src/setup.py respects --prefix, --install-base, --root, --user #21573
Description
... rather than relying on some of the SAGE_LOCAL-derived environment variables.
Comments from src/Makefile.in (before removal in #29411):
## We poison all environment variables that have paths to the sage source and build directories,
## but keep the directories below SAGE_LOCAL intact.
##
## TODO: Do also something about the SAGE_LOCAL stuff:
## - some of it (our install) should be communicated only through --install-base;
## - others (installed packages that we pull in) through some configuration mechanism.
##
## TODO: Currently providing --install-base=$(SAGE_LOCAL) leads to this error:
## [sagelib-7.4.beta4] running install
## [sagelib-7.4.beta4] error: install-base or install-platbase supplied, but installation scheme is incomplete
See also:
- Testsuite for src/setup.py #21678 Testsuite for src/setup.py
Depends on #30371
CC: @kiwifb @timokau @jhpalmieri @isuruf @antonio-rojas
Component: build
Issue created by migration from https://trac.sagemath.org/ticket/21573