gitlab-ci: Replace YAML anchors by extends (native_build_job)

'extends' is an alternative to using YAML anchors
and is a little more flexible and readable. See:
https://docs.gitlab.com/ee/ci/yaml/#extends



Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210418233448.1267991-3-f4bug@amsat.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>