Update render_site.yml

[MicheleTobias]

From Tyler: if you are using GitHub Actions to compile your reader, be advised that renv now throws an error when it can't find a lockfile (#1474). Most readers (if not all) do not use lockfiles, so this will prevent a successful Action run. I've commented out the line in the render_site.yml template so that readers default to no renv install, but you'll need to do the same for any of your readers. Alternatively, you can set up a lockfile