We make SAGE_ROOT and SAGE_LOCAL optional. If one is unset, we skip setting the various environment variables that depend on it.

This change hopefully simplifies distribution packaging, with the goal of allowing unmodified scripts src/bin/sage and src/bin/sage-env to be shipped.

See also:

• src/bin/sage-env: Make sure $SAGE_VENV/bin is at the beginning of the PATH #30013 src/bin/sage-env: Make sure SAGE_SCRIPTS_DIR is at the beginning of the PATH
• Install sage-env-config as part of sage_conf #29850 Install sage-env-config as part of sage_conf; make sage-env-config optional
• Meta-ticket Meta-ticket: Split sage-env into 5 to clean up sage configuration #21707: Split sage-env into 5 to clean up sage configuration

CC: @kiwifb @orlitzky @antonio-rojas @isuruf @tobihan @jhpalmieri @jamesjer

Component: scripts

Author: Matthias Koeppe

Branch/Commit: c8e6910

Reviewer: Dima Pasechnik, Jerry James

Issue created by migration from https://trac.sagemath.org/ticket/29951