pin checkout to trigger commit SHA, not branch HEAD

Use github.sha instead of github.ref so in-flight sweep jobs
don't pick up new commits pushed to the branch mid-run.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: cquil11

.github/workflows/benchmark-multiturn-tmpl.yml

@@ -128,7 +128,7 @@ jobs:
         with:
           token: ${{ secrets.REPO_PAT }}
           fetch-depth: 0
-          ref: ${{ inputs.ref || github.ref }}
+          ref: ${{ inputs.ref || github.sha }}
           submodules: true
 
       - name: Launch job script