* If `/tmp` is a `tmpfs` or somehow not the same filesystem, `os.Rename()` will fail so use `io.Copy()` instead
* don't defer cleanup of `to` as this removes the newly-created file if the operation is successful, making `install` and `upgrade` _functionally_ `uninstall`
* defer cleanup of the temporary file since it will still be in place if `os.Rename()` fails
Fixes: #130