arrow

MINOR: [R][CI] use Renviron.site for setting `MAKEFLAGS`

According to https://cran.r-project.org/doc/manuals/r-devel/R-admin.html#Customizing-package-compilation `R_HOME/etc/Makevars.site` (or `Makevars` under `~/.R/`) should work, but when trying that I was unable to get R to recognize that (e.g. the Makevars file would exist, with the appropriate line in it), but `Sys.getenv("MAKEFLAGS")` would return nothing.

Renviron.site, does, however work!

Closes #11394 from jonkeane/parallel-build-r

Authored-by: Jonathan Keane <jkeane@gmail.com>
Signed-off-by: Jonathan Keane <jkeane@gmail.com>