dispatcher: Replace broken use of apt-add-repository

The latest LAVA images use deb822-style repository descriptions and
apt-add-repository doesn't work with this style. Replace it with a
direct change to /etc/apt/sources.list.d/debian.sources that enables
non-free repos.

This also means we can retire the double update/install since we no
longer need to install software-properties-common just to enable
the non-free repo.

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>