ci: Rename build.sh -> jobs.sh

ci: Rename build.sh -> jobs.sh

After the recent changes, this script no longer executes any logic
anymore, it merely defines the jobs run in the GitLab environment. In
order to use it, one has to source the file in the environment and then
run one of the job "functions". For that, the 'build.sh' name is no
longer descriptive enough and 'jobs.sh' feels more suitable and less
misleading.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>