Use poetry version in env, use for cache key

.github/workflows/test.yaml

@@ -9,6 +9,9 @@ permissions:
   checks: write
   pull-requests: write
 
+env:
+  POETRY_VERSION: 1.8.4
+
 jobs:
   test:
     runs-on: ubuntu-latest
@@ -25,7 +28,7 @@ jobs:
       - name: Install Poetry
         uses: snok/install-poetry@v1
         with:
-          version: 1.8.4
+          version: ${{ env.POETRY_VERSION }}
           virtualenvs-create: true
           virtualenvs-in-project: true
 
@@ -33,8 +36,8 @@ jobs:
         id: cached-pip-wheels
         uses: actions/cache@v4
         with:
-          key: venv-${{ steps.setup-python.outputs.python-version }}-${{ hashFiles('**/poetry.lock') }}
           path: .venv  # While ~/.cache is a fine default, I want to separate this cache from other caches
+          key: venv-${{ steps.setup-python.outputs.python-version }}-${{ env.POETRY_VERSION }}-${{ hashFiles('**/poetry.lock') }}
 
       - name: Install dependencies
         run: cd backend && poetry install --no-interaction --no-root