Add apt-get update before installing system packages in CI

The CI was failing because the runner's package index was stale,
causing a 404 when fetching libasound2-dev (a dependency of
portaudio19-dev). Running apt-get update first refreshes the index.

.github/workflows/coverage.yaml

@@ -29,6 +29,7 @@ jobs:
 
       - name: Install system packages
         run: |
+          sudo apt-get update
           sudo apt-get install -y portaudio19-dev
 
       - name: Install dependencies