get branch-name from github var (#69)

.github/workflows/trigger-prow-build-job-reusable.yml

@@ -63,6 +63,6 @@ jobs:
           GITHUB_REPO: ${{ github.event.repository.name }}
         with:
           context: "${{ inputs.CONTEXT }}"
-          commit_ref: "release-${VERSION}" # the name of the release branch.
+          commit_ref: ${{ GITHUB_REF_NAME }} # the name of the release branch.
           timeout: ${{ inputs.TIMEOUT }}
           check_interval: ${{ inputs.INTERVAL }}