Update RTD config to have poetry properly use RTD venv (#942)

.readthedocs.yml

@@ -8,11 +8,9 @@ build:
   jobs:
     post_create_environment:
       - pip install poetry
-      - poetry config virtualenvs.create false
     post_install:
-      # Install dependencies with 'docs' dependency group
-      # https://python-poetry.org/docs/managing-dependencies/#dependency-groups
-      - poetry install --with docs
+      # Instructs Poetry to use RTD venv
+      - VIRTUAL_ENV=$READTHEDOCS_VIRTUALENV_PATH poetry install --with docs
 
 sphinx:
   configuration: docs/conf.py