GH Actions: report CS violations in the PR

The cs2pr tool will allow to display the results from an action run in checkstyle format in-line in the PR code view, which should improve usability of the workflow results.

This implements this for the style check command.

Includes switching the PHP version to PHP 7.4, as PHP 8.0 is not fully supported yet in PHP_CodeSniffer (full support expected in PHPCS 3.6.0).

Ref: https://github.com/staabm/annotate-pull-request-from-checkstyle

Author: jrfnl

.github/workflows/test.yml

@@ -17,9 +17,9 @@ jobs:
       - name: Setup PHP
         uses: shivammathur/setup-php@v2
         with:
-          php-version: '8.0'
-          extensions: json
+          php-version: '7.4'
           coverage: none
+          tools: cs2pr
 
       - name: Checkout code
         uses: actions/checkout@v2
@@ -28,7 +28,11 @@ jobs:
         uses: ramsey/composer-install@v1
 
       - name: Run code sniffer
-        run: vendor/bin/phpcs
+        continue-on-error: true
+        run: vendor/bin/phpcs --report-full --report-checkstyle=./phpcs-report.xml
+
+      - name: Show PHPCS results in PR
+        run: cs2pr ./phpcs-report.xml
 
   bundle:
     name: Bundle binary