Allow to configure location of boilerplate and input/output files

[king-of-poppk]

As a follow-up question to #261, I'd like to be able to use bubblewrap to isolate the different callr subprocesses coming from the same parent process. Currently it seems all callr::r calls will reference the same .so files and read inputs and write outputs to the same temporary folder.

Would it be possible to store the reused files in a separate folder (so that that one can be mounted read-only), and the input/output files in another folder so that that one can be mounted read-write separately?

Additionally would it be possible to have an easy way to know and/or configure where those are located?

There are probably cross-platform concerns there too maybe, although I am not sure whether many folders vs one is so big of a deal.