fix: Install poetry-plugin-export plugin in pipeline

Author: czubocha

.github/workflows/validate.yml

@@ -61,7 +61,7 @@ jobs:
         run: python -m pip install --force setuptools wheel
 
       - name: Install pipenv / poetry
-        run: python -m pip install pipenv poetry
+        run: python -m pip install pipenv poetry && poetry self add poetry-plugin-export
 
       - name: Install serverless
         run: npm install -g serverless@${{ matrix.sls-version }}