provide reference ref for benchmark action

it's apparently not possible to also provide a PR number for the comment
to target, since that is automically determined by the action (and wrong
for workflow_run workflows)

See also: benchmark-action/github-action-benchmark#250

.github/workflows/post-benchmark.yml

@@ -48,3 +48,4 @@ jobs:
           # Enable job summary
           summary-always: true
           comment-always: true
+          ref: ${{ github.event.workflow_run.head_sha }}