[CI][NFC] Add manual dispatch to test workflow for PR #19280 (#19292)

Forgot to add manual dispatch in #19281

Author: ianayl

.github/workflows/sycl-nightly-benchmarking.yml

@@ -1,7 +1,11 @@
 name: SYCL Nightly Benchmarking
 
 # Placeholder workflow for testing https://github.com/intel/llvm/pull/19280
+on:
+  workflow_dispatch:
 
 jobs:
   do-nothing:
     runs-on: ubuntu-latest
+    steps:
+      - run: echo "Test workflow for 19280"