Providing option to override temp location would be a great enhancement

[scottporter]

Our server cluster is set up so each server has a small /tmp partition. When I need to save out large temporary files, I do so by manually specifying another partition when I create the temporary files. I don't see any facility in callr for specifying a different location other than the one returned by tempdir(). If I understand correctly, I would have to start R by specifying a different location either in .Renviron or using an environment variable. Our R setup works fine for our regular use, but I started testing out callr using future.callr and quickly brought a server to its knees and incurred the wrath of our sys admins. With some convincing I can get the setup of our cluster changed, but it would be nice to have control over the temp location callr uses from inside R (instead of having to set it before R starts). However, not sure if that's even possible based on how each callr session starts.