Instead of having to rely on external repositories (like the
ubuntu-cloud-archive/yoga-staging PPA for the debhelper v13 backport),
let's use the pkg script instead. The pkg scripts ensure that the
debian/ directory looks as expected for the according distribution. So
for executing coverity + unit-tests in the Ubuntu/focal / ubuntu-20.04
environment on GitHub, let's use pkg/deb/backports/focal for generating
the according Debian source tree.
Related change in pkg: make `wrap-and-sort` execution optional. We don't
have devscripts installed by default in Ubuntu/focal / ubuntu-20.04
environment on GitHub, while usage of wrap-and-sort can be considered
optional for our purposes.
Change-Id: Ic252b323408fab557e3f151a93f61aa8fac1a30f