Feature request: when using Conda to install R, add the ability to specify the version of R that Conda installs

[DilumAluthge]

This should be relatively straightforward to implement, e.g. something like this in the deps/build.jl file:

Rhome = get(ENV, "R_HOME", "")
if Rhome == "*"
    r_version = get(ENV, "RCALL_CONDA_R", ">=3.4.0,<4")
    Conda.add("r-base$(r_version)")
end

Then, you can override the version of R installed by Conda by setting RCALL_CONDA_R, e.g.:

ENV["RCALL_CONDA_R"] = ">=3.6.0,<4"

ENV["RCALL_CONDA_R"] = "=3.5"

[palday]

I'm marking this as stale/not planned, because we now have experimental CondaPkg support, where this can be specified via CondaPkg's dependency file.

That said, if somebody wants to open a PR with the necessary changes, I would be happy to revirew.