Add build and dist/ freshness check to CI

.github/workflows/unit-tests.yml

@@ -27,5 +27,16 @@ jobs:
       - name: Install dependencies
         run: npm ci
 
+      - name: Build
+        run: npm run build
+
+      - name: Check dist/ is up-to-date
+        run: |
+          if [ -n "$(git status --porcelain)" ]; then
+            echo "::error::dist/ is out of date. Run 'npm run build' and commit the changes."
+            git status --short
+            exit 1
+          fi
+
       - name: Run tests
         run: npm test