Add a new CI job to the common pipeline which triggers a run of
renovatebot for the CI pipeline's project if the
TRIGGER_RENOVATEBOT_ENABLED variable is set.

Since no jobs are added *unless* that variable is set, this ensures that
renovatebot runs are always explicitly triggered.

The auto-devops/trigger-renovatebot.yml template also includes a
template CI job which can be used to construct trigger jobs with more
elaborate rulesets.