fix: Check tar file completeness before download

Before this commit, the process_pre_job_server_download() function would
check if the necessary tar file exists on the remote server. But, because it is
 written there by sftp, and sftp doesn't generate any lock file or temp files, in-progress sftp writes look like they are available and the job continues. With this change, the job will only process if the tar file is completely written.