Added production build test to CI

- we recently had an issue where a dependency update broke the
  release process because it was building in production mode, but our CI
  never ran in this mode and we missed it
- this commit adds a test to run in prod mode so hopefully we catch it
  next time

ghost/admin/.github/workflows/test.yml

@@ -14,8 +14,23 @@ on:
       - master
       - 'renovate/*'
 jobs:
+  prod-build:
+    runs-on: ubuntu-18.04
+    if: github.event_name == 'push' || (github.event_name == 'pull_request' && !startsWith(github.head_ref, 'renovate/'))
+    name: Production Build
+    steps:
+      - uses: actions/checkout@v2
+      - run: yarn
+      - run: grunt shell:ember:prod
+
+      - uses: daniellockyer/action-slack-build@master
+        if: failure() && github.event_name == 'push' && github.ref == 'refs/heads/master'
+        with:
+          status: ${{ job.status }}
+        env:
+          SLACK_WEBHOOK_URL: ${{ secrets.SLACK_WEBHOOK_URL }}
   test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     if: github.event_name == 'push' || (github.event_name == 'pull_request' && !startsWith(github.head_ref, 'renovate/'))
     strategy:
       matrix: