allow for external site folder via environment variable

[zkamvar]

This Pull request will fix #536 by allowing users to specify an experimental SANDPAPER_SITE environment variable to be the path to a folder where the site should be rendered.

To install this PR, you can use {pak}:

pak::pkg_install("carpentries/sandpaper#537")

To test this out, navigate to a lesson you want to test and open R, running:

tmp <- tempfile()
Sys.setenv("SANDPAPER_SITE" = tmp)
sandpaper::serve()

You should see the site building into the temporary folder.

In BASH, you can use the following:

TMP=$(mktemp -d)
SANDPAPER_SITE=${TMP} R -q -e 'sandpaper::serve(quiet = FALSE)'

[zkamvar]

This finally passes tests and I have confirmed with the author of the original issue that it works on their end, so I am happy to merge this.