fix stale ffmpeg ref

Author: speediedan

.github/workflows/ci_testing.yml

@@ -39,6 +39,7 @@ jobs:
 
       - name: Install dependencies
         run: |
+          sudo apt-get update --fix-missing
           sudo apt install -y ffmpeg
           pip --version
           pip install --requirement requirements.txt --find-links https://download.pytorch.org/whl/cpu/torch_stable.html