ci: Use Poetry 2 for updating

This fixes an issue with virtualenv 20.31.0 and Poetry 1, see:
python-poetry/poetry#10378

Author: robin-nitrokey

.github/workflows/ci.yaml

@@ -155,7 +155,7 @@ jobs:
       - name: Install required packages
         run: apt update && apt install -y ${REQUIRED_PACKAGES}
       - name: Install Poetry
-        run: pip install "${POETRY_SPEC}"
+        run: pip install "poetry==2"
       - name: Create virtual environment
         run: make install
       - name: Update locked dependencies