Meta-ticket: Set environment for subprocesses invoked by Sage

[mkoeppe]

Instead of relying on running in an environment set up by sage-env, we should set up specific environment variables whenever we invoke executables. This will be configurable through sage_conf.

This is so that Sage is fully functional even when not being run from within sage-env. This continues what was done in #29038/#30563 for Maxima.

As this ticket removes some hard-coded paths depending on SAGE_LOCAL from sagelib, it will reduce the need for patching for downstream packaging.

A possible implementation strategy goes through sage.features.Executable.

This is part of:

• #21707 Meta-ticket: Split sage-env into 5 to clean up sage configuration

Taken care of in separate tickets:

• #32036 sage.env: Remove fallback of SAGE_LOCAL to SAGE_VENV, remove direct uses of SAGE_LOCAL from the Sage library
• #31296 sage.features.Executable: Prepend $SAGE_VENV/bin:$SAGE_LOCAL/bin to PATH, remove dependency on distutils
• #31275 qepcad
• #29024 singular
• #29644 gap
• #30887 4ti2
• environment for C compiler use at runtime (includes ARCHFLAGS - see #31227)
• #31041 environment for sage.misc.cython

[mkoeppe]

Hoping we can make progress on this ticket this week - ​https://wiki.sagemath.org/days111

[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.