Add gh commit status for periphery

Author: RafaelKayumov

.buildkite/pipeline.yml

@@ -83,6 +83,9 @@ steps:
         command: ".buildkite/commands/run-periphery.sh"
         depends_on: build
         plugins: [$CI_TOOLKIT]
+        notify:
+          - github_commit_status:
+              context: Periphery - Detect unused code
 
       - label: 🧹 Lint Translations
         command: gplint /workdir/WooCommerce/Resources/AppStoreStrings.pot