Fix incorrectly detecting legacy ostree-push

Before, detection would just run inside the outer GitLab pipeline rules. However, this was actually incorrect all along: generate_pipeline.py has its own entirely separate logic to determine what Docker image to use for running the build, which meant that the pipeline itself could end up assuming a different ostree-push version than was actually being used.

Instead, this moves the detection logic exclusively into generate_pipeline.py, and the outer pipeline only sets a regex used to match the releases with legacy ostree-push.

https://phabricator.apertis.org/T9516

Signed-off-by: Ryan Gonzalez ryan.gonzalez@collabora.com