CI: Do the CMake native debug build from an Autotools `make dist`

CI: Do the CMake native debug build from an Autotools `make dist`

We officially release dbus in the form of Autotools `make dist` tarballs,
but people who have downloaded those tarballs should be able to choose
the CMake build system. Our CI should assert that they can.

(The Autotools debug build already does a `make distcheck`, which
asserts that tarball releases can be used for an Autotools build.)

Signed-off-by: Simon McVittie <smcv@collabora.com>
Resolves: dbus#255