Update worker command and slicer build location

[naglepuff]

Recently tried using this repository to bootstrap some new Histomics development and noticed a couple of issues getting up and running.

First, the command girder_worker no longer exists and so I've changed the docker compose service to just invoke celery directly instead.

Also, the slicer cli plugin directory, was renamed (girder_slicer_cli_web -> slicer_cli_web), so the init.sh script was failing when trying to build, as that script has not updated with the new location.