Ensure CBS workspace is a child folder of build root directory #336
Description
To help ensure that pipelines using the Common Back-end Script (CBS) templates run within an appropriate scope, the workspace option should be limited to specifying a folder underneath the path defined by the buildRootDir property (in pipelineBackend.config)/$PIPELINE_WORKSPACE environment variable.
This implies dropping support for specifying absolute paths via the workspace option, which was primarily used for testing purposes earlier.