Fix ci on windows-latest: issue with pip upgrade (#2264)

* Update unit-tests.yml

* Add user

* Update unit-tests.yml

* Remove user arg

.github/workflows/unit-tests.yml

@@ -57,7 +57,7 @@ jobs:
       - name: Get pip cache dir
         id: pip-cache
         run: |
-          pip install -U pip
+          pip install -U pip || python -m pip install -U pip 
           echo "::set-output name=dir::$(pip cache dir)"
 
       - uses: actions/cache@v2