ci: Detach the commit body of assign-pr-number (#3309)

Co-authored-by: Joongi Kim <[email protected]>
Co-authored-by: octodog <[email protected]>

.github/workflows/assign-pr-number.yml

@@ -14,7 +14,7 @@ jobs:
     steps:
     - uses: actions/checkout@v4
       with:
-        ref: ${{ github.head_ref}}
+        ref: ${{ github.head_ref }}
         fetch-depth: 2
         lfs: false
         token: ${{ secrets.WORKFLOW_PAT }}
@@ -46,7 +46,8 @@ jobs:
         git config user.name "$author_name"
         if [ -n "$(git diff --staged)" ]; then
           git commit \
-            -m "docs: Rename the news fragment with the PR number\n\n${{ join(fromJSON(steps.assign_pr_number.outputs.rename_results), '\n') }}" \
+            -m 'docs: Rename the news fragment with the PR number' \
+            -m $'${{ join(fromJSON(steps.assign_pr_number.outputs.rename_results), '\n') }}' \
             --author="$author_name <$author_email>" \
             --trailer "Co-authored-by: octodog <[email protected]>"
           git push

changes/3309.misc.md

@@ -0,0 +1 @@
+Fix the commit message format when assigning the PR number to an anonymous news fragment