TT#124273 GitHub actions: enable Ubuntu PPA for debhelper backport

ubuntu-20.04 + ubuntu-latest (currently also pointing at 20.04) don't
provide debhelper v13 (required for debhelper-compat (= 13)), therefore
our workflow fails with:

| The following packages have unmet dependencies:
|  builddeps:. : Depends: debhelper-compat (= 13)
| E: Unable to correct problems, you have held broken packages.

So enable the ubuntu-cloud-archive/yoga-staging PPA,
which provides a backport of debhelper v13:
https://launchpad.net/~ubuntu-cloud-archive/+archive/ubuntu/yoga-staging/+packages
and fixes our failing unity + coverity builds on github.

While at it, using apt-get with `-qq` displays only the following
message on package installation problems:

| E: Unable to correct problems, you have held broken packages.

Whereas with `-q`, we get the actual underlying problem, being:

| The following packages have unmet dependencies:
|  builddeps:. : Depends: debhelper-compat (= 13)
| E: Unable to correct problems, you have held broken packages.

Change-Id: I67439a8f2f27e902710af43ff52599c8a58a5f98

.github/workflows/coverity.yml

@@ -28,8 +28,10 @@ jobs:
 
       - name: Get build dependencies
         run: |
-          sudo apt-get install -qq -y --no-install-recommends git ca-certificates curl
-          sudo apt-get build-dep -qq -y -Ppkg.ngcp-rtpengine.nobcg729 .
+          # note: debhelper v13 isn't available from ubuntu-20.04 yet, we need a backport
+          sudo apt-add-repository -y ppa:ubuntu-cloud-archive/yoga-staging
+          sudo apt-get install -q -y --no-install-recommends git ca-certificates curl
+          sudo apt-get build-dep -q -y -Ppkg.ngcp-rtpengine.nobcg729 .
 
       - name: Get coverity scan
         run: |

.github/workflows/unit-tests.yml

@@ -21,7 +21,9 @@ jobs:
 
       - name: Get build dependencies
         run: |
-          sudo apt-get build-dep -qq -y -Ppkg.ngcp-rtpengine.nobcg729 .
+          # note: debhelper v13 isn't available from ubuntu-20.04 yet, we need a backport
+          sudo apt-add-repository -y ppa:ubuntu-cloud-archive/yoga-staging
+          sudo apt-get build-dep -q -y -Ppkg.ngcp-rtpengine.nobcg729 .
 
       - name: Run unit tests with sanitizers enabled
         run: |