use custom version of package dependencies in setup-r-dependencies?

[kevinushey]

For renv, CI builds are failing with R 3.6 on Windows due to a failure to build RcppTOML from sources:

  i Building RcppTOML 0.2.2
  x Failed to build RcppTOML 0.2.2
  Error: 
  ! error in pak subprocess
  Caused by error in `stop_task_build(state, worker)`:
  ! Failed to build source package RcppTOML.
  ---
  Backtrace:
  1. pak::lockfile_install(".github/pkg.lock")
  2. pak:::remote(function(...) { ...
  3. err$throw(res$error)

Unfortunately I don't see anything else in the build log, but I'm guessing it's because RcppTOML requires C++17 features not available with the older version of Rtools used by R 3.6?

What's the right way to declare that I want to use an older version of RcppTOML for R on Windows 3.6?

For reference: https://github.com/rstudio/renv/blob/b89e5d1fc86de9de5068f095f25049fac0db3885/.github/workflows/R-CMD-check.yaml#L55-L58