I installed Rtools following the official guide (installing as well as adding the .Renviron file with the PATH).
Rstudio, however, doesn't recognise the installation:
> devtools::find_rtools()
WARNING: Rtools is required to build R packages, but the version of Rtools previously installed in C:/rtools40 has been deleted.
Rtools is installed and seems to work:
Set-up: Windows 10, RStudio 1.2.5042, R 4.0.0, RTools 4.0