TT#108003 GH coverity workflow: ensure to use up2date repository

Package installations might fail, if the underlying repository changed
(as compared to the running base image).

Let's match behavior of .github/workflows/unit-tests.yml.

Fixes https://github.com/sipwise/rtpengine/runs/4751441960:

| E: Failed to fetch http://azure.archive.ubuntu.com/ubuntu/pool/main/s/systemd/libsystemd-dev_245.4-4ubuntu3.13_amd64.deb  404  Not Found [IP: 52.252.75.106 80]

Change-Id: Iaca7086d68ac8047ef22357636f1124ab9148c59

.github/workflows/coverity.yml

@@ -22,6 +22,10 @@ jobs:
       - name: Checkout source
         uses: actions/checkout@v2
 
+      - name: Update APT cache
+        run: |
+          sudo apt-get update
+
       - name: Get build dependencies
         run: |
           sudo apt-get install -qq -y --no-install-recommends git ca-certificates curl