install ffmpeg in github action

Author: Bomme

.github/workflows/main.yml

@@ -16,6 +16,8 @@ jobs:
         uses: actions/setup-python@v2
         with:
           python-version: ${{ matrix.python }}
+      - name: Install ffmpeg
+        run: sudo apt install -y ffmpeg
       - name: Install tox and any other packages
         run: pip install tox
       - name: Run tox