gitlab-ci: when processing trigger, check out correct Yaru revision before calling snapcraft

The initial version of the trigger support failed due to Snapcraft being unable to find the source-commit: I inserted into the snapcraft.yaml. While the ephemeral PR auto-merge commits exist in the Github repository, they aren't transferred by a simple git clone.

So instead I now manually check out Yaru and set the head revision to the desired commit. I then modify the snapcraft.yaml to use this local checkout.