Supply RSTUDIO_PANDOC to packaging tasks, including R Markdown render

[jmcphers]

Several beta users have already reported trouble rendering documents with Positron because the render tasks can't find Pandoc, even though it ships with Positron.

This change supplies theRSTUDIO_PANDOC environment variable when running packaging tasks and R Markdown rendering jobs. It's done using the same logic we use to supply RSTUDIO_PANDOC to the main R session.

Addresses #3776.

QA Notes

The embedded Pandoc is only used when there's no other Pandoc on the $PATH. To test this, uninstall all your Pandocies (make sure which pandoc returns nothing) first so that only the bundled Pandoc is available.

This also touches the code that injects Pandoc into the main R session, so make sure e.g. rmarkdown::pandoc_version() still works there.

[juliasilge]

I realized I couldn't try this out with just a dev build since that's not when the bundling happens 🙈 so I triggered a release build, nuked all my pandocies, and tested out the render command as well as rmarkdown::pandoc_available(). Looks good! 👍

We'll want to take a similar approach to #2723 for the R pkg testing command but we'll need a different approach for #3842.