GH Actions: harden the workflow against PHPCS ruleset errors

If there is a ruleset error, the `cs2pr` action doesn't receive an `xml` report and exits with a `0` error code, even though the PHPCS run failed (though not on CS errors, but on a ruleset error).

This changes the GH Actions workflow to allow for that situation and still fail the build in that case.

.github/workflows/cs.yml

@@ -39,8 +39,9 @@ jobs:
 
       # Check the code-style consistency of the PHP files.
       - name: Check PHP code style
-        continue-on-error: true
+        id: phpcs
         run: vendor/bin/phpcs -- --report-full --report-checkstyle=./phpcs-report.xml
 
       - name: Show PHPCS results in PR
+        if: ${{ always() && steps.phpcs.outcome == 'failure' }}
         run: cs2pr ./phpcs-report.xml