GH Actions: use the xmllint-validate action runner

Instead of doing all the installation steps for xmllint validation in the workflow, use the ✨ new dedicated `phpcsstandards/xmllint-validate` action runner instead.

Notes:
* This does mean that the `bin/xml-lint` script is now no longer used in CI.
    I've left the script in place as there may be contributors who want to run these checks locally and the script can help them with that, though happy to remove the script if nobody uses it.

Ref: https://github.com/marketplace/actions/xmllint-validate

Author: jrfnl

.github/workflows/basics.yml

@@ -22,6 +22,9 @@ jobs:
     name: 'Basic CS and QA checks'
     runs-on: ubuntu-latest
 
+    env:
+      XMLLINT_INDENT: '	' # This is a tab character.
+
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
@@ -33,21 +36,6 @@ jobs:
           coverage: none
           tools: cs2pr
 
-      # Updating the lists can fail intermittently, typically after Microsoft has released a new package.
-      # This should not be blocking for this job, so ignore any errors from this step.
-      # Ref: https://github.com/dotnet/core/issues/4167
-      - name: Update the available packages list
-        continue-on-error: true
-        run: sudo apt-get update
-
-      - name: Install xmllint
-        run: 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
-      - name: Enable showing XML issues inline
-        uses: korelstar/xmllint-problem-matcher@v1
-
       # Validate the composer.json file.
       # @link https://getcomposer.org/doc/03-cli.md#validate
       - name: Validate Composer installation
@@ -65,8 +53,19 @@ jobs:
           # Bust the cache at least once a month - output format: YYYY-MM.
           custom-cache-suffix: $(date -u "+%Y-%m")
 
-      - name: 'Validate XML against schema and check code style'
-        run: ./bin/xml-lint
+      # Validate the XML file.
+      - name: Validate rulesets against schema
+        uses: phpcsstandards/xmllint-validate@v1
+        with:
+          pattern: "./*/ruleset.xml"
+          xsd-file: "vendor/squizlabs/php_codesniffer/phpcs.xsd"
+
+      # Check the code-style consistency of the XML file.
+      # Note: this needs xmllint, but that will be installed via the phpcsstandards/xmllint-validate action runner in the previous step.
+      - name: Check XML code style
+        run: |
+          diff -B --tabsize=4 ./WordPressVIPMinimum/ruleset.xml <(xmllint --format "./WordPressVIPMinimum/ruleset.xml")
+          diff -B --tabsize=4 ./WordPress-VIP-Go/ruleset.xml <(xmllint --format "./WordPress-VIP-Go/ruleset.xml")
 
       # Check the code-style consistency of the PHP files.
       - name: Check PHP code style