10.13, 10.14, 10.15 installers don't work

[ryandesign]

The installer isos this script creates for 10.13, 10.14, and 10.15 don't work. I need to compare the installers this script creates with those that Apple's createinstallmedia creates. As a last resort, this script could punt on supporting 10.13-10.15 and just call createinstallmedia directly but ideally I'd like this script to be able to do it itself.