gitlab-ci: Replace build_script -> step_script in Cirrus jobs

gitlab-ci: Replace build_script -> step_script in Cirrus jobs

Long due upgrade, see [1]:

  In GitLab Runner 13.2 a translation for step_script to
  build_script was added to the custom executor. In 14.0
  the build_script stage will be replaced with step_script.

We are using GitLab 17 [2]!

This removes the following warning:

  WARNING: Starting with version 17.0 the 'build_script'
  stage will be replaced with 'step_script':
  https://gitlab.com/groups/gitlab-org/-/epics/6112

[1] https://about.gitlab.com/releases/2021/05/22/gitlab-13-12-released/#remove-translation-from-stepscript-to-buildscript-in-custom-executor
[2] https://about.gitlab.com/releases/2024/05/16/gitlab-17-0-released/

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20240816213203.18350-1-philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>