Handle better xelatex missing on non TinyTeX installation

[cderv]

From: #7676, TinyTeX is not available on ARM so Quarto can sometimes be used with OS package manager texlive.

As shared

From #909:

xelatex is indeed the default (but also I believe comes with all versions of tinytex and all downloads of texlive so isn't normally something that should require dynamic download).

Though it could happen that xelatex is not available, like on Debian where this is needed for installation

sudo apt-get update
sudo apt-get install texlive texlive-xetex

We should try detect that and advice

Though we should watch out for xelatex not found error due to problem in LaTeX installation detection, and not really missing xelatex command in a found installation of TeX Live.