jupyter - use different name for input .ipynb file

[cscheid]

Closes #4802

[cderv]

I did not see this PR at the time and I commented in #9689 about possibly implication for other part of our code.

Also keep-ipynb: true will keep the intermediates, but it will be a index.quarto_ipynb with unrecognized extension.

What is unsafe to consider <fileStem>.quarto.ipynb for the name so the the extension is kept ?

Anyhow, I am surprise we don't have issues in our test, but Iplaces where we handle *.ipynb as intermediates like #9689 also needs adaptation