Fix Ubuntu FFmpeg installation in CI

- Replace FedericoCarboni/setup-ffmpeg action with native apt-get for Ubuntu
- Keep setup-ffmpeg action only for Windows where it works reliably
- Use brew for macOS and apt-get for Ubuntu for better reliability
- Fixes TypeError: fetch failed on Ubuntu runners with setup-ffmpeg@v3

Author: Alexander Refsum Jensenius

.github/workflows/ci.yml

@@ -32,8 +32,14 @@ jobs:
       run: |
         brew install ffmpeg
 
-    - name: Install FFmpeg on Ubuntu/Windows
-      if: runner.os != 'macOS'
+    - name: Install system dependencies on Ubuntu
+      if: runner.os == 'Linux'
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y ffmpeg
+
+    - name: Install FFmpeg on Windows
+      if: runner.os == 'Windows'
       uses: FedericoCarboni/setup-ffmpeg@v3
       id: setup-ffmpeg