Ensure versions of sandpaper and varnish can be pinned

[zkamvar]

One thing that was brought up by @tobyhodges is the concept that people like to be able to tweak the template. At the moment, this is not explicitly possible, but it is possible without too much effort by piggybacking on {pkgdown}'s approach to allowing custom templates a la tidytemplate: https://tidytemplate.tidyverse.org/ (which is exactly what we did with {varnish}).

Another aspect is the fact that some instructors like to tweak the lessons by forking the official lessons. We want to make sure that they don't have to go through any changes to the site if they don't want to.

Both of these aspects can be achieved by introducing a mechanism by which users can lock the versions of sandpaper and varnish needed to build the site.

[zkamvar]

Note: this has been partially acheived in https://github.com/carpentries/actions/releases/tag/v0.8

[zkamvar]

This was fixed in #270