ci: Move the test execution to a separate step

.github/workflows/run_tests.yml

@@ -17,4 +17,4 @@ jobs:
           python_blender_executable="$(pytest-blender --blender-executable $blender_executable)"
           $python_blender_executable -m ensurepip
           $python_blender_executable -m pip install pytest
-          pytest --blender-executable $python_blender_executable tests/
+      - run: pytest tests/