Make assets image build on stable branches

What does this MR do and why?

Part of gitlab-com/gl-infra/delivery#2674 (closed)

Currently we do a trigger to the cng-mirror repository to build images on every commit to a stable branch, because we want to deploy the images built into a stable environment (via trigger to release-environments).

However, the rules on when we build the assets image (which we need) vs when we trigger cng-mirror and release-environments are different.

This causes a failure when we haven't got the assets image tag available to use to build images and thus a subsequent deploy also fails (normally job dependencies would stop the deploy due to images failing to build, but as these jobs are in allow_failure: true this doesn't currently happen).

This change adds the specific rule about building on stable branches (that we use to trigger the whole release-environments child pipeline) to the rules for the build-assets-image job. This means whenever we would deploy to a release-environment, we are guaranteed to have run the build-assets-image job.

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