specify rtd venv path env var

.readthedocs.yaml

@@ -9,7 +9,7 @@ build:
       - pip install poetry
       - poetry config virtualenvs.create false
     post_install:
-      - poetry install --only base,main,docs
+      - VIRTUAL_ENV=$READTHEDOCS_VIRTUALENV_PATH poetry install --only base,main,docs
 
 sphinx:
   configuration: docs/source/conf.py