Preliminary steps to save the CI infrastructure

[kwankyu]

To improve the situation with the CI infrastructure, this PR:

• added comments untangling obscure code in CI-related files, for those poor guys who ever attempt to read the files for whatever reasons.

• while doing the cosmetic changes, a bug (about -uninstall targets) was found build/make/Makefile.in, which is fixed here.

to test, do

$ ./configure --enable-dot2tex | grep dot2tex
$ make build | grep dot2tex
$ ./configure --disable-dot2tex | grep dot2tex
$ make build | grep dot2tex

• fixed some jobs in the CI-linux workflow that fail because of duplicate artifact names.

• removed "gentoo" from the list of the default systems that CI runs for. This is how to properly modify the list:

  • first edit tox.ini
  • run tox -e update_docker_platforms
  • commit the changes

• "optional" and "experimental" jobs now run upon "standard" docker images, instead of "maximal" ones, to avoid "out of runner space" error.

test CI run:

https://github.com/kwankyu/sage/actions/runs/13333439284

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

[github-actions]

Documentation preview for this PR (built with commit 3cfeeca; changes) is ready! 🎉
This preview will update shortly after each push to this PR.

[dimpase]

before proceeding it's good to decide whether we rather go with the other PR