Eliminate direct use of os.getenv in sage.interfaces

[mkoeppe]

... as shown by git grep getenv, environment variables such as SAGE_MACAULAY2_COMMAND are checked.

These should go through variables defined in sage.env (and perhaps through sage.feature, Executable.absolute_filename(), #31296) so that they can be configured using sage_conf as well.

(see also #32867 comment:31 to #32867 comment:36)

See also: #32957 Replace have_program usage by Features

• Depends on sage.features.Executable: Prepend $SAGE_VENV/bin:$SAGE_LOCAL/bin to PATH #31296
• Depends on FriCAS spkg-configure and Feature #35838

CC: @kiwifb @antonio-rojas @fchapoton @orlitzky @tornaria @dimpase

Component: refactoring

Issue created by migration from https://trac.sagemath.org/ticket/33405

[mkoeppe]

Dependencies: #31296