test(CHANGELOG): add GH action step summaries from check

Author: ErichDonGubler

.github/workflows/changelog.yml

@@ -37,4 +37,4 @@ jobs:
 
       - name: Run `cargo xtask changelog …`
         run: |
-          cargo xtask changelog --emit-github-messages "origin/${{ github.event.pull_request.base.ref }}"
+          cargo xtask changelog --emit-github-messages "origin/${{ github.event.pull_request.base.ref }}" >> "$GITHUB_STEP_SUMMARY"