.gitlab-ci.yml: enable sysbuild when building nRF sample

Sysbuild handles mcuboot build, signatures and flashing with a lot less effort, by handling multi-image builds more cohesively and automatically.

Ref: https://docs.zephyrproject.org/latest/build/sysbuild/index.html

Signed-off-by: Zygmunt Krynicki zygmunt.krynicki@huawei.com