CI: Don't install weak dependencies (Recommends)

CI: Don't install weak dependencies (Recommends)

In particular this avoids installing the TeX toolchain.

However, this also means we don't install dbus, which broke some tests
in minimal containers where dbus wasn't already installed, because the
messagebus user wouldn't have been created. Make sure that user exists,
using the same adduser call as the Debian dbus package.

CMake really wants to find a C++ compiler (even though we only use C++
when compiling for Windows), so explicitly install the default
version of the GNU C++ compiler, g++.

Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit cb6abc63919d6f84f634aee94d20c53d2759c382)