Add R to build workflows

[wildintellect]

Now that we're starting to write R documentation in qmd files we need R to be installed in the build workers for the render to work.

Otherwise you'll get an error

ERROR: Error executing 'Rscript': No such file or directory (os error 2)

It doesn't matter if you freeze or eval:false the dependency is there even if you don't execute code blocks.

Documentation for what we need is https://github.com/quarto-dev/quarto-actions/blob/main/examples/example-03-dependencies.md

[wildintellect]

So I read the docs more closely https://quarto.org/docs/publishing/github-pages.html#freezing-computations

1. If we enable freeze, then executed code (qmd not sure about ipynb) is stored in a _freeze directory that needs to be pushed to git. This will mean for R code contributors will be responsible for executing a quarto render before they can push changes to code in qmd files.

2. The alternative is to add R to the action env with required packages.

Which would we prefer?

[jsignell]

I think the freeze option is fine. This is already essentially what we do with the ipynb files.