TT#111150 force update of APT cache on github

Recent builds fail with

Run sudo apt-get build-dep -qq -y -Ppkg.ngcp-rtpengine.nobcg729 .
E: Failed to fetch http://azure.archive.ubuntu.com/ubuntu/pool/main/s/systemd/libsystemd-dev_245.4-4ubuntu3.6_amd64.deb  404  Not Found [IP: 52.154.174.208 80]
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?
E: Failed to process build dependencies

Change-Id: If7d4d0be41bbc93b7e131fd1e68094a9a374a6f2

.github/workflows/unit-tests.yml

@@ -18,6 +18,10 @@ 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 build-dep -qq -y -Ppkg.ngcp-rtpengine.nobcg729 .