copr-keygen: better backup script
    
    We need to have YYYY-MM-DD in file name to actually have correct
    incremental backups.  Since we don't overwrite the old backup files now
    and we have different filename each day, let's remove the old backup
    files and keep only the last one (this is to mimic what happens with DB
    backups on copr frontend, which runs
    roles/postgresql_server/files/backup-database).
    
    While on it, let's double-quote variable uses.
    
    Per advice in:
    https://pagure.io/fedora-infrastructure/issue/8904