Use github.sha as the reference commit hash in the modern distributio…

…n CI

.github/workflows/modern-distributions.yml

@@ -30,5 +30,5 @@ jobs:
             --manifest distributions/DistributionInfo.json \
             --github-token '${{ secrets.GITHUB_TOKEN }}' \
             --github-pr '${{ github.event.pull_request.number }}' \
-            --github-commit '${{ github.event.pull_request.merge_commit_sha }}'
+            --github-commit '${{ github.sha }}'
           shell: bash