gitlab-ci: Only use one process in Windows jobs for compilation

gitlab-ci: Only use one process in Windows jobs for compilation

The Windows jobs are currently aborting at weird places - and
there's the suspicion that it's due to memory constraints in
the Windows containers. Let's switch to single-threaded compilation
to decrease the pressure on the memory load, and to make the
job more deterministic for further investigations.

Message-Id: <20220825193323.104768-1-thuth@redhat.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>