kpy save timing could be optimized

[pvandyken]

kpy save first writes the venv to a tmp file, then moves it to the final destination. This helps prevent data loss when overwriting an old venv: should the process get interrupted, the overwritten file won't get deleted until the last possible moment.

This last possible moment could be delayed even further: currently, the tmp tar file is being written on whatever tmp filesystem is available. This could be a different filesystem the the storage dir, which would require a cp operation upon moving. This cp could take a minute or two if the tar is very large. A better approach would be to first mv the tar file into the dest directory under a tmp name, then delete the overwritten file, then rename the tmp file to final