Try to auto-detect that we need tinytex

[gaborcsardi]

We can't really auto-detect when it is needed for R CMD check, because that depends on the check options. But we can possibly auto-detect it when it is needed for vignettes and/or for quarto.

Cf. r-lib/pkgdown#2757 (comment)

[gaborcsardi]

Or maybe check-r-package could auto-install tinytex?

[gaborcsardi]

setup-tinytex takes at least 1-2 minutes to install on Windows (and right now it takes 5-6 until [1] is fixed), so we definitely don't want to auto-install it.

[1] rstudio/tinytex#504