CI: set python-version in volk-gnsssdr-windows job

.github/workflows/main.yml

@@ -193,6 +193,8 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v4
+        with:
+          python-version: '3.12'
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip