Pandoc fails to find echo bib when ./check is run: we have no way of
telling Pandoc (via the pandoc::Pandoc structure), what it's working
directory should be. I am about to introduce changes that will mean
Subplot will fail in this case.

This will all be sorted when we switch from using Pandoc to
cmark_pulldown for parsing Markdown, and when we do, this commit
should be reverted, but that change is too big for me at this time.

Sponsored-by: author