CI: add apt-get update

Author: mgeier

.github/workflows/main.yml

@@ -23,6 +23,7 @@ jobs:
     - name: Install JACK on Ubuntu
       if: startsWith(matrix.os, 'ubuntu')
       run: |
+        sudo apt-get update
         sudo apt-get install ${{ matrix.jack }}
     - name: Install JACK on macOS
       if: startsWith(matrix.os, 'macos')