gitlab: use template for ubuntu-24.04-s390x jobs

gitlab: use template for ubuntu-24.04-s390x jobs

Most of the test is pure boilerplate so to save ourselves from
repetition move all the main bits into a minimal copy of
native_build_job_template but without the caching.

We keep all the current allow_fail, manual and configure setups but do
take the opportunity to replace the inline nproc calls to using a
common JOBS variable. We also fix the namespace check to use the
QEMU_CI_UPSTREAM variable.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20251117115523.3993105-14-alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>