Make sure R/Rscript exits

[sckott]

The original code from JosiahParry https://gist.github.com/JosiahParry/94d5b72430c688ff742b7652e78c1343 (thank you 🙏) was not exiting the R subprocess - it just hung after completing running tests.

After some googling, added some bits and bobs after spawn and it now exits after completing https://github.com/sckott/rt-cli/blob/main/src/rscript.rs

Anything else to do on this front? Only tested on macOS