try to force use of python 3.8 for windows as required by craft

Signed-off-by: Matthieu Gallien <matthieu.gallien@nextcloud.com>

.github/workflows/windows-build-and-test.yml

@@ -19,6 +19,9 @@ jobs:
         with:
           fetch-depth: 0 # Shallow clones should be disabled for a better relevancy of analysis
 
+      - uses: actions/setup-python@v1
+        with:
+          python-version: '3.8'
       - name: Install Craft Master with Nextcloud Client Deps
         shell: pwsh
         run: |