Use PULL_BASE_REF in release-notes-check (#2847)
Use PULL_BASE_REF in release-notes-check (#2847)

* Use PULL_BASE_REF in release-notes-check

Instead of base branch. Prow reuses the name master when running
the job. Using the SHA is the way to go here.

* Fix variable name
istio/test-infra,istio/test-infra,istio/test-infra,istio/test-infra