... only set it in the GH Actions workflows by setting MAKE or USE_MAKEFLAGS.

This will make it less surprising for manual use.

Before merging this, we need to remember to update ci-sage.yml of upstream packages!

CC: @jhpalmieri

Component: porting

Author: Matthias Koeppe

Branch/Commit: u/mkoeppe/tox_ini__build_bin_write_dockerfile_sh__do_not_use__make__k__by_default @ 4aede69

Issue created by migration from https://trac.sagemath.org/ticket/31574