attempt to fix doc build on readthedocs (#1556)

readthedocs build is failing https://app.readthedocs.org/projects/sceptre/

Author: zaro0508

.readthedocs.yaml

@@ -3,18 +3,17 @@ version: 2
 build:
   os: "ubuntu-22.04"
   tools:
-    python: "3.10"
+    python: "3.12"
   jobs:
-    post_create_environment:
+    post_install:
       # 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 extras dependency group
+      # Install dependencies with 'docs' dependency group
       # https://python-poetry.org/docs/managing-dependencies/#dependency-groups
-      - poetry install --all-extras
+      # 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 --all-extras
 
 sphinx:
   configuration: docs/_source/conf.py