The command `kde-builder workspace` tries to install this project, but the README.md states that > It is not recommended to install this on your system — you should use a virtual machine instead. Installing this on real hardware will leave behind files not trivially uninstallable and could leave your system in a non-function state. Seems to me it is an oversight since the latest workspace is supposed to run on real machines. Thankfully the installer failed on my machine. Until this is cleared up I will use `kde-builder workspace --ignore-projects plasma-login-manager` because I test exclusively on real hardware.
Installing this in a prefix (as when using kde-builder) is fine, and the install step succeeds now.