gitlab-ci.yml: Avoid recompiling the sources in the test jobs

Currently, our check-system-* jobs are recompiling the whole sources
again. This happens due to the fact that the jobs are checking out
the whole source tree and required submodules again, and only try
to use the "build" directory with the binaries and object files as an
artifact from the previous stage - which simply does not work right
anymore (with the current version of meson). Due to some changed
time stamps, meson/ninja are always trying to rebuild the whole tree.

In the long run, we could likely use "meson test --no-rebuild", but
there is still some work going on in that area to improve the user
experience. So until this has been done, simply avoid recompiling the
sources with a trick: pass NINJA=":" to the make process in the test
jobs. Also check out the submodules manually before updating the
timestamps in the build folder, so that the binaries are definitely
newer that all the source files.
This saves ca. 10 - 15 minutes of precious CI cycles in each run.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210126065757.403853-1-thuth@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

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