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.

As the CS run is run in every test run in the matrix, passing the results on to CS2PR would result in violations being displayed multiple times, so using conditions to only pass the CS violations on to the PR for one of the runs.

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

.github/workflows/test.yml

@@ -66,6 +66,7 @@ jobs:
           php-version: ${{ matrix.php }}
           ini-values: ${{ steps.set_ini.outputs.PHP_INI }}
           coverage: none
+          tools: cs2pr
 
       # Install dependencies and handle caching in one go.
       # @link https://github.com/marketplace/actions/install-composer-dependencies
@@ -97,9 +98,18 @@ jobs:
         run: vendor/bin/phpunit tests/AllTests.php --no-configuration --bootstrap=tests/bootstrap.php --dont-report-useless-tests
 
       - name: 'PHPCS: check code style without cache, no parallel'
-        if: ${{ matrix.custom_ini == false }}
+        if: ${{ matrix.custom_ini == false && matrix.php != 7.4 }}
         run: php bin/phpcs --no-cache --parallel=1
 
+      - name: 'PHPCS: check code style to show results in PR'
+        if: ${{ matrix.custom_ini == false && matrix.php == 7.4 }}
+        continue-on-error: true
+        run: php bin/phpcs --no-cache --parallel=1 --report-full --report-checkstyle=./phpcs-report.xml
+
+      - name: Show PHPCS results in PR
+        if: ${{ matrix.custom_ini == false && matrix.php == 7.4 }}
+        run: cs2pr ./phpcs-report.xml
+
       - name: 'Composer: validate config'
         if: ${{ matrix.custom_ini == false }}
         run: composer validate --no-check-all --strict