[GitHub Actions] set shell to poetry

.github/workflows/testing_external_plugins.yaml

@@ -47,6 +47,7 @@ jobs:
 
     - name: Install dependencies
       run: |
+        python -m pip install pip --upgrade
         pip install poetry
         poetry config --local virtualenvs.in-project true
         poetry install --remove-untracked
@@ -59,7 +60,9 @@ jobs:
     - name: Test with pytest
       if: success() || failure()
       run: poetry run pytest --junitxml=reports/pytest/results.xml
+      shell: poetry shell {0}
 
     - name: Test with behave
       if: success() || failure()
       run: poetry run behave --no-skipped --format progress2 --junit --junit-directory reports/behave
+      shell: poetry shell {0}