Consider setting Pandoc version in GHA

[jennybc]

If I do this, I guess I'd need to do it across all workflows? Here's how it's done:

- uses: r-lib/actions/setup-pandoc@v1
  with:
    pandoc-version: '2.13'

It feels like reprex should be testing itself with the Pandoc version currently shipping with RStudio. Probably the version shipping with the preview version of the IDE. That might happen via the action (r-lib/actions#336) and/or I could do it myself.