GH Actions/CS: fix build failure

Fix failing installation of the xmllint tooling.

Author: jrfnl

.github/workflows/cs.yml

@@ -46,7 +46,9 @@ jobs:
         uses: "ramsey/composer-install@v2"
 
       - name: Install xmllint
-        run: sudo apt-get install --no-install-recommends -y libxml2-utils
+        run: |
+          sudo apt-get update
+          sudo apt-get install --no-install-recommends -y libxml2-utils
 
       # Show XML violations inline in the file diff.
       # @link https://github.com/marketplace/actions/xmllint-problem-matcher