I don't know what you have in the variable $software_depot, but when I do for example the following it works for me.
Add-EsxSoftwareDepot-DepotUrlhttps://hostupdate.vmware.com/software/VUM/TEST/main/vmw-depot-index.xml
I am on PowerCLI 6.5.4, which includes the same VMware.ImageBuilder module you are using.
