feat: support triggering dry-runs from pipelines

What

Allow triggering dry-runs if TRIGGER_DRY_RUN = true is set.

Why

Currently (before this MR) dry-runs could only be triggered from the service repo itself or for testing runwayctl changes themselves, but you couldn't trigger a dry-run by using trigger from the service repository as required by the topology service.

Sadly there's no way to figure out if a pipeline is local to the service repository or coming from being triggered (pipeline source is the same) so sadly we have to use a new environment variable.

Related MR: gitlab-org/cells/topology-service!32 (merged)

/cc @daveyleach