<File>K:\OLAP Database Backups\SSAS**OLAP_test.abf**</File> <AllowOverwrite>true</AllowOverwrite>
That <File> parameter is the backup file and it is a fix name in your XMLA command. Unfortunately you can't make it dynamically in a job step, you have to find an other backup solution, like:
https://sqlpowershell.blog/2013/06/05/ssas-cube-backup-strategies-and-step-by-step-approach-to-setup-and-configure-backup/
http://www.ssas-info.com/analysis-services-scripts/2679-script-to-backup-ssas-database-and-delete-old-backup-files