Use temp directory for intermediate files.

[jashapiro]

We will need the report rendering to use a temporary directory for intermediate files, as the package directory that is the source for the template may not be writeable.

Looking at https://rmarkdown.rstudio.com/docs/reference/render.html it looks like we will need to specify the intermediate_dir and the most logical thing will be to make that a temp directory.

Originally posted by @jashapiro in AlexsLemonade/scpca-nf#208 (comment)