Sys.which('make') returns an empty vector, resulting in installation failure of package built with .cpp

Question

I've already updated r, rtools and rstudio. But it seems unable to find make.exe, while in C:\rtools40\usr\bin\make.exe it exists.

> find_rtools()
[1] TRUE
> Sys.which('make')
make 
  "" 
> R.Version()$version.string
[1] "R version 4.0.3 (2020-10-10)"

Rtools is rtools40-x86_64.exe downloaded from https://cran.r-project.org/bin/windows/Rtools/. All packages are up to date (Rstudio told me so).

And when I'm trying to install a package built with Rcpp, it returns

Warning in system(cmd) : 'make' not found
ERROR: compilation failed for package 'testpkg'

and the installation fails. Is there any way I can deal with it?