fix nightly build on linux (#461)

Author: xadupre

.azure-pipelines/linux-CI-nightly.yml

@@ -56,6 +56,7 @@ jobs:
   - script: |
       python -c "import onnxconverter_common"
       python -c "import onnxruntime"
+      pip install -e .
       pytest tests --ignore=tests/sparkml --doctest-modules --junitxml=junit/test-results.xml
     displayName: 'pytest - onnxmltools'