update caching workflow; use universe

[zkamvar]

This updates the workflow and depends on the success of carpentries/sandpaper-docs#32

The following changes are made:

• Creation of cache restore files now comes before the cache mechanism
• I have switched from our drat repository to our R universe because it builds every hour
• I have removed the Install mac deps step because we have added {textshaping}, {gert}, and {stringi} to our dependencies.

This will really speed up our build process!

[zkamvar]

This appears to be working... kind of, so I'm going to merge it.

[zkamvar]

Note: The kind of part was due to me expecting that we could restore the packages discovered in the dependency query step. It turns out that this creates a loop where you are forever installing packages.

The reason for this is that the cache is restored based on the key which is controlled by the R version and the output of remotes::package_deps(), which tells you what packages need to be updated based on the current state of packages in your library. We query the dependencies before the cache is restored, so this gives us a snapshot of ALL the packages we need.

My thought was that I would be able to use that object downstream to update the packages, but because it queried the packages when we didn't have anything in the library, it would cause a situation where ALL of the packages would be updated. The solution (in b3cbea8) was to re-query the the packages AFTER the cache was restored to get an accurate assessment of what packages (if any) needed updating.