Update `register_runner_spec.rb` tag to `runner-staging
Justification
As discussed in #1533 (comment 1269369537), this spec is a candidate to be removed since all the other tests that require running a pipeline already registers a runner to work. As of this writing, there are 35 other specs that all set up runner to perform the respective tests.
Updated: Instead of removing the spec, this spec is being updated to use the runner-staging
tag that will allow it to run in an upcoming runner-staging
environment.
Edited by Richard Chong