test: Fix mirroring test by skipping CI on push

The test pipeline is now failing with:

INFO:root:testing mirroring from apertis/v2024dev3 to downstream/v2024dev3
INFO:root:checking out the old known tag apertis/0.5.10.2-5co2
INFO:root:rolling back apertis/v2024dev3 to apertis/0.5.10.2-5co2
INFO:root:disable CI on tests/dash while we roll back branches and tags
INFO:root:re-enable CI on tests/dash
INFO:root:trigger mirroring pipeline on apertis/v2024dev3
INFO:root:monitoring pipeline https://gitlab.apertis.org/tests/dash/-/pipelines/592570
AssertionError: mirroring pipeline https://gitlab.apertis.org/tests/dash/-/pipelines/592570 didn't succeed: failed

And the failing dash pipeline ends with:

Attempt merging apertis/v2024dev3 into downstream/v2024dev3 via proposed-updates/apertis/v2024dev3/ba7650dc
branch 'apertis/v2024dev3' set up to track 'origin/apertis/v2024dev3'.
Auto-merging debian/apertis/copyright
CONFLICT (content): Merge conflict in debian/apertis/copyright
Automatic merge failed; fix conflicts and then commit the result.
source package dash
running git branch -f apertis/v2024dev3 origin/apertis/v2024dev3
downstream downstream/v2024dev3 0.5.10.2-5co2
upstream apertis/v2024dev3 0.5.12-2+apertis1

This is because a new "Refresh the automatically detected licensing information" commit gets added to the downstream/v2024dev3 branch by the CI pipeline created when pushing it. We disable CI on the whole project before doing exactly to avoid this kind of issues, but the GitLab/Gitaly state synchronization is often racy and we may end up with spurious pipelines being created.

To avoid that we can use the ci.skip push option to prevent more reliably the creation of new pipelines.