Update merge-to-main script (#1420)

* Before creating the merge-main PR, branch it off main, so merge conflicts can be addressed in the branch rather than by committing to main.

* Format.

Author: jonsimantov

.github/workflows/update-feature-branches.yml

@@ -17,6 +17,8 @@ env:
   defaultBranchPattern: "feature_branch/*"
   defaultMainBranch: "main"
   triggerTestsLabel: "tests-requested: quick"
+  branchPrefix: "workflow/auto-merge-feature-branch-"
+
 
 jobs:
   list_feature_branches:
@@ -85,6 +87,11 @@ jobs:
           python scripts/gha/install_prereqs_desktop.py
           python -m pip install requests
 
+      - name: Name new branch
+        run: |
+          date_str=$(date "+%Y%m%d-%H%M%S")
+          echo "NEW_BRANCH=${{env.branchPrefix}}${{github.run_number}}-${date_str}" >> $GITHUB_ENV
+
       - name: Create merge PR
         id: create-pr
         run: |
@@ -115,7 +122,10 @@ jobs:
 
             > Created on ${date_str} by [${{github.workflow}} workflow]($GITHUB_SERVER_URL/$GITHUB_REPOSITORY/actions/runs/$GITHUB_RUN_ID).
             "
-            pr_number=$(python scripts/gha/create_pull_request.py --token ${{ steps.generate-token.outputs.token }} --base "${{ matrix.branch_name }}" --head "${main_branch}" --title "${pr_title}" --body "${pr_body}")
+            git checkout main
+            git checkout -b "${NEW_BRANCH}"
+            git push --set-upstream origin "${NEW_BRANCH}"
+            pr_number=$(python scripts/gha/create_pull_request.py --token ${{ steps.generate-token.outputs.token }} --base "${{ matrix.branch_name }}" --head "${NEW_BRANCH}" --title "${pr_title}" --body "${pr_body}")
             echo "created_pr_number=${pr_number}" >> $GITHUB_OUTPUT
 
       - name: Set test trigger label.