If options("repos") (see options()) contains an entry called
"CRAN", then that is returned. If it is a list, it is converted
to a character vector.
default_cran_mirror()A named character vector of length one, where the
name is "CRAN".
Otherwise the RStudio CRAN mirror is used.
default_cran_mirror()
#> CRAN
#> "https://cloud.r-project.org"