Toolchain-update PR: move debugging hints to extra comment

This will stop them from appearing in default commit messages.

.github/workflows/toolchain-upgrade.yml

@@ -36,6 +36,7 @@ jobs:
         run: git clean -f
 
       - name: Create Pull Request
+        id: create_pr
         if: ${{ env.next_step == 'create_pr' }}
         uses: peter-evans/create-pull-request@v6
         with:
@@ -47,14 +48,26 @@ jobs:
             Update Rust toolchain from nightly-${{ env.current_toolchain_date }} to
             nightly-${{ env.next_toolchain_date }} without any other source changes.
 
-            This is an automatically generated pull request. If any of the CI checks fail,
-            manual intervention is required. In such a case, review the changes at
-            https://github.com/rust-lang/rust from
-            https://github.com/rust-lang/rust/commit/${{ env.current_toolchain_hash }} up to
-            https://github.com/rust-lang/rust/commit/${{ env.next_toolchain_hash }}. The log
-            for this commit range is:
+      - name: Add debugging hints
+        if: ${{ steps.create_pr.outputs.pull-request-number }}
+        uses: actions/github-script@v6
+        with:
+          script: |
+            github.rest.issues.createComment({
+              issue_number: ${{ steps.create_pr.outputs.pull-request-number }},
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              body: >
+                This is an automatically generated pull request. If any of the CI checks fail,
+                manual intervention is required. In such a case, review the changes at
+                https://github.com/rust-lang/rust from
+                https://github.com/rust-lang/rust/commit/${{ env.current_toolchain_hash }} up to
+                https://github.com/rust-lang/rust/commit/${{ env.next_toolchain_hash }}. The log
+                for this commit range is:
+
+                ${{ env.git_log }}
+            })
 
-            ${{ env.git_log }}
       - name: Create Issue
         if: ${{ env.next_step == 'create_issue' }}
         uses: dacbd/create-issue-action@main