Set Java Temp Directory
Java sets the java temp directory to `/tmp` by default. However, this is usually not desirable in R. As a result, this function allows changing that behavior. Further, this function is fired on package load to ensure all temp files are written to the R temp directory.
set_java_tmp_dir(tmp_dir = tempdir())
optional. The new temp directory. Defaults to the R temp directory
On package load, we use `getOption("xlsx.tempdir", tempdir())` for the default value, in case you want to have this value set by an option.
The previous java temp directory (prior to any changes).