use mkdir -p because directory exists on repeated builds

[pomi601]

This minor change adds -p to the mkdir command so it does not fail if the directory already exists. I ran into this when attempting repeated builds using R CMD INSTALL on an extracted directory containing the release version of FastRWeb. This change corrects the problem.

I've tested that mkdir -p works in PowerShell and CMD on whatever version of Windows 11 I am running, in case that's a concern. I wasn't sure if this script was meant to be cross platform.