Update window-app.yml

.github/workflows/window-app.yml

@@ -24,7 +24,7 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         pip install wheel
-        if exist requirements.txt pip install -r requirements.txt
+        if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
     - name: Lint with flake8
       run: |
         # Stop the build if there are Python syntax errors or undefined names