Switch back to installing using pip instead of poetry

There have been multiple complaints about how GWCelery is deployed. I don't yet understand all of the nuance of the discussion, but have heard from at least one member of our review team that they will not approve a release that includes an editable installation in the deployment process. I believe this MR fixes these concerns.

EDIT: Here is a helpful list of limitations of editable installations: https://setuptools.pypa.io/en/latest/userguide/development_mode.html#limitations