CI: run 'apt-get update' before installing pkgs

Fix that installing packages can fail with 404 errors if the package
index inside the container image is outdated.

Author: osmith42

.github/workflows/test.yaml

@@ -21,6 +21,7 @@ jobs:
 
     - name: Install dependencies (apt)
       run: |
+        sudo apt-get update
         sudo apt-get -q install -y \
           libsctp-dev \
           mariadb-server \