Template issues with R lessons on Windows

[maxim-belkin]

CI shows that there are some issues with R lessons on Windows (link) specifically with the "Install needed packages" step:

Run install.packages(setdiff(c('remotes', 'rprojroot', 'renv', 'desc', 'rmarkdown', 'knitr'), rownames(installed.packages())))
Installing packages into 'D:/a/_temp/Library'
(as 'lib' is unspecified)
Error in contrib.url(repos, "source") : 
  trying to use CRAN without setting a mirror
Calls: install.packages -> contrib.url
Execution halted
Error: Process completed with exit code 1.

This was introduced in ed77eda.
CC @fmichonneau