Create method to rebuild lesson material on CI if build process changes

[zkamvar]

One of the things that's different about the new version of sandpaper+varnish is that we now use a new variable in the pkgdown yaml. The problem with this approach is that the pkgdown yaml is only ever rebuilt when there are files that need to be rebuilt (because this was how the schedule was historically built, which is no longer the case).

The problem is if we are implementing a new feature that affects the build process, we want that to be triggered on the lessons even if none of the source files have changes.