Reference: go/librarian:cli-reimagined

When Librarian is already running within Docker, this is a value of the form : where host-mount is a mount point from the perspective of the Docker host, and local-mount is how that same mount point appears within Docker (i.e. where Librarian will see it).

When this is specified, Librarian must validate that any mount points it provides to language containers are within this local-mount, and replace mount sources to refer to host-mount, so that the language container "sees" the right directory.

When running in Kokoro jobs, this is expected to be specified as ${KOKORO_HOST_ROOT_DIR}:${KOKORO_ROOT_DIR}

This change should remove the need for sourcing this information from the environment variables that are sourced today, so lets remove that logic at the same time.