Set RHOME env-var to account for Rscript

[mhermher]

Rscript is not portable because it uses a compile time absolute path for the R home directory. So any R code that calls something like system('/path/to/Rscript', ...) will fail because the Rscript file will in turn call R at the original hard-coded absolute path. For example, parallel::newPSOCKnode() uses this type of command, meaning that any code using PSOCK parallel cluster will not work.

I am not sure if shiny or any of this electron setup ever calls code that in turn makes a call to Rscript. If not, then it is probably fine.

Interestingly, Rscript does check to see if the R home directory is being overridden in the environment, and uses that path instead of the compile time path. However, it does so by looking for the deprecated RHOME env-var instead of R_HOME. Instead of re-compiling Rscript (and wrangling with all the tooling involved in that), we can work around it by assigning into the RHOME env-var as well.

I actually have not tested your library here, but I was reading through it because I was trying to create a portable R as well. I see that you have a binary Rscript in the root directory, so maybe this is a re-compiled version that works so maybe my change is not needed at all.

You can test it simply by ./R-Portable-Mac/bin/Rscript -e "R.home()" and see if that is correctly set. Without this current fix, my attempt at portable R was giving me the compile time path rather than the ported path.

[mhermher]

Sorry if you have already accounted for this. I wanted to type that all out for my own documentation at least. Ignore this PR if you've already resolved this in some other way.