TT#124273 GitHub actions: switch packaging for unit-tests to Ubuntu jammy

So far we used ubuntu-latest for the unit-tests, though this is no
longer using Ubuntu/focal (wich we used to build for) but Ubuntu/jammy
(22.04 LTS, AKA Jammy Jellyfish) nowadays.

Instead let's stick to ubuntu-22.04 since we also use a specific release
for the packaging part.

FTR: see https://github.com/actions/runner-images for the overview
of available GitHub Actions Runner Images

Change-Id: I41f2fe1c35bccb40f1f8df59b23446ab27ce1033

.github/workflows/unit-tests.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     name: Run unit tests
 
     steps:
@@ -21,12 +21,12 @@ jobs:
 
       - name: Get build dependencies
         run: |
-          echo "Generating Debian source for usage on Ubuntu/focal / ubuntu-20.04"
+          echo "Generating Debian source for usage on Ubuntu/jammy / ubuntu-22.04"
           cd pkg/deb/
           ./generator.sh
-          ./backports/focal
+          ./backports/jammy
           rm -rf debian
-          mv focal debian
+          mv jammy debian
           echo "Installing Debian packages"
           sudo apt-get build-dep -q -y -Ppkg.ngcp-rtpengine.nobcg729 .