fix installation after poetry change

Author: MelvinKl

.github/workflows/ci.yaml

@@ -76,6 +76,8 @@ jobs:
           python-version: ${{ matrix.python-version }}
       - name: Install core        
         run: |
+          pip install poetry
+          poetry config virtualenvs.create false
           git clone https://github.com/stackitcloud/stackit-sdk-python-core.git core
           cd core;make install-dev;
       - name: Lint