The error message "WorkspaceInstaller is not supposed to be executed in databricks runtime" indicates that the WorkspaceInstaller class or function is not designed to be run directly within a Databricks runtime environment. This is a general limitation of the Databricks Labs installation process, not specific to UCX.

From the context provided, it appears that the WorkspaceInstaller is intended to be used in a different environment, possibly a local development setup or a different execution context outside of the Databricks runtime.

Can you try to install CLI in your local machine and test it from there?