gitlab-ci: Fix warnings about multiple pipelines

GitLab is currently quite unhappy about the rules:when without an if and it is printing a bunch of warnings like the one below on each pipeline page:

jobs:submit-tests-ostree-armhf-target-uboot may allow multiple
pipelines to run for a single action due to rules:when clause with
no workflow:rules - read more:
https://docs.gitlab.com/ee/ci/troubleshooting.html#pipeline-warnings

I've chosen to not use workflow:rules because the goal of the templates is to make them reusable, and it would be unfortunate to have them trigger some warnings if you include them with no other special provision.