ci: pin the setup-ffmpeg action to its specific sha

Author: joanise

.github/workflows/test.yml

@@ -23,7 +23,7 @@ jobs:
           python-version: "3.8"
           cache: "pip"
 
-      - uses: FedericoCarboni/setup-ffmpeg@v2
+      - uses: FedericoCarboni/setup-ffmpeg@583042d32dd1cabb8bd09df03bde06080da5c87c # v2
 
       - name: Install dependencies and package
         run: pip install -e . mypy coverage