Check out merge commit before creating patch (#2171)

Author: JoshMock

.github/workflows/serverless-patch.yml

@@ -29,7 +29,7 @@ jobs:
       - uses: actions/checkout@v4
         with:
           repository: elastic/elasticsearch-js
-          ref: main
+          ref: '${{ github.event.pull_request.merge_commit_sha }}'
       - name: Generate patch file
         run: |
           git format-patch -1 --stdout ${{ github.event.pull_request.merge_commit_sha }} > /tmp/patch.diff