Install PostCSS in CI

Author: oleg-nenashev

.github/workflows/ci.yml

@@ -33,6 +33,9 @@ jobs:
           # https://github.com/actions/setup-node/blob/main/docs/advanced-usage.md#caching-packages-data
           cache-dependency-path: '**/package-lock.json'
 
+      - name: Install PostCSS
+        run: npm install postcss-cli
+
       - name: Build
         run: hugo --minify