chore(github): add build stage slack message

Author: Seolhun

.github/workflows/build-stage.yml

@@ -37,3 +37,9 @@ jobs:
       - name: Local Test
         if: ${{ always() }}
         run: npm run lh
+
+      - name: Slack Notification
+        uses: rtCamp/action-slack-notify@v2
+        env:
+          SLACK_CHANNEL: github-builds
+          SLACK_WEBHOOK: ${{ secrets.SLACK_WEBHOOK }}