Fix RTD poetry configuration

This follows a change in RTD's user guide found at https://docs.readthedocs.io/en/stable/build-customization.html#install-dependencies-with-poetry, which they changed in readthedocs/readthedocs.org#11152 following a breaking change introduced by Poetry 1.8.

Author: teutoburg

.readthedocs.yaml

@@ -13,12 +13,12 @@ build:
       # Install poetry
       # https://python-poetry.org/docs/#installing-manually
       - pip install poetry
-      # Tell poetry to not use a virtual environment
-      - 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
+      # VIRTUAL_ENV needs to be set manually for now.
+      # See https://github.com/readthedocs/readthedocs.org/pull/11152/
+      - VIRTUAL_ENV=$READTHEDOCS_VIRTUALENV_PATH poetry install --with docs
 
 sphinx:
   configuration: docs/source/conf.py