speedbench-al: default open-pr to false (artifact-only by default)

Make the workflow default to Option 1 (upload the AL matrix as an
artifact for manual review/paste) rather than auto-opening a PR. The
auto-PR path stays available as an opt-in (open-pr: true), but keeping
it off by default avoids exposing a write-scoped PAT on the self-hosted
runner and matches the repo's artifact-collection convention.

Author: qiching

.github/workflows/speedbench-al.yml

@@ -49,10 +49,10 @@ on:
         type: string
         default: '480'
       open-pr:
-        description: "Open a PR updating benchmarks/speedbench-reference-al.yaml"
+        description: "Open a PR updating benchmarks/speedbench-reference-al.yaml (default off: artifact-only, paste values in manually)"
         required: false
         type: boolean
-        default: true
+        default: false
       ref:
         description: "Git ref (branch/sha) to checkout"
         required: false