Make sure src/setup.py respects --prefix, --install-base, --root, --user

[mkoeppe]

... 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:

• #21678 Testsuite for src/setup.py

[mkoeppe]

Moving some tickets to 9.2. This is not a promise that I will be working on them.

[mkoeppe]

Sage development has entered the release candidate phase for 9.3. Setting a new milestone for this ticket based on a cursory review of ticket status, priority, and last modification date.