Need to set VIRTUALENV for readthedocs to install mkdocs dependencies.

.readthedocs.yaml

@@ -15,14 +15,13 @@ 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:
       # Configure git authentication
       - git config --global credential.helper store
       - echo "https://${GH_TOKEN}:@github.com" > $HOME/.git-credentials
-      # Install packages
-      - poetry -C .config/mkdocs/ install
+      # VIRTUAL_ENV needs to be set manually for now.
+      # See https://github.com/readthedocs/readthedocs.org/pull/11152/
+      - VIRTUAL_ENV=$READTHEDOCS_VIRTUALENV_PATH poetry -C .config/mkdocs/ install
     pre_build:
       # Symlink all files to a .docs directory so we can build outside the root
       - ln -sfT . .docs