ci: register the S3 benchmark workflow for dispatch from its branch

A workflow_dispatch workflow can only be dispatched once GitHub has
registered it, which happens on the default branch or when one of its
triggers fires. A push trigger limited to this branch registers it; validate
runs only for workflow_dispatch, so a push run skips every job.

Signed-off-by: jeremyhi <fengjiachun@gmail.com>

.github/workflows/oswal-s3-bench.yml

@@ -1,5 +1,8 @@
 name: Object Store WAL S3 Benchmark
 on:
+  # Registers the workflow for dispatch before it reaches the default branch; such runs skip every job.
+  push:
+    branches: [bench/oswal-s3-wal]
   workflow_dispatch:
     inputs:
       smoke:
@@ -57,6 +60,7 @@ env:
   JANITOR_TTL_HOURS: ${{ inputs.janitor_ttl_hours }}
 jobs:
   validate:
+    if: github.event_name == 'workflow_dispatch'
     runs-on: ubuntu-latest
     timeout-minutes: 15
     outputs: