TT#151550 github workflow: run on ubuntu-latest with PPA

ubuntu-20.04 doesn't provide debhelper-compat (= 13), therefore fails with:

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

Let's switch from ubuntu-20.04 to ubuntu-latest, which
currently still points to ubuntu-20.04, but should reduce
our maintenance efforts.

Furthermore enabled 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 coverity builds on github.

Change-Id: I3bba166843f164b67b90c403cc772dfd939eeae7

.github/workflows/coverity.yml

@@ -11,7 +11,7 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
 
     env:
       COVERITY_SCAN_TOKEN: ${{ secrets.COVERITY_SCAN_TOKEN }}
@@ -28,6 +28,8 @@ jobs:
 
       - name: Get build dependencies
         run: |
+          # 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 .