running update_cache() in subfolder causes a new renv/ folder to be created

[zkamvar]

In sarahkaspar/biostatistics-course#1 I had encountered a situation where the user attempted to run sandpaper::update_cache() while inside of the episodes/ folder (which is not an uncommon use case) and instead of fixing their issue, they accidentally committed the updated cache and all of the symbolic links to episodes/renv/

When update_cache() is run, it should first use root_path() to make sure it's in the correct part of the lesson