Use a more robust path for R

[TimTaylor]

This ensure that the version of R called matches the one you are in. Mainly relevant when you have multiple versions of R installed (e.g. R and R-devel).

I've not yet tested the windows command yet (no access) so marking as draft for now.

[otoomet]

Adding here a note that rocker/r-devel docker image also contains both R and R-devel (called "RD"). So it is relevant if you want to run tests in a docker container.