GH Actions: update PHP ini configuration

Add `display_startup_errors=On` as per the current recommendation from PHPUnit.

Ref: sebastianbergmann/phpunit-documentation-english@b3b159c

Author: jrfnl

.github/workflows/ci.yml

@@ -78,7 +78,7 @@ jobs:
         uses: shivammathur/setup-php@v2
         with:
           php-version: ${{ matrix.php }}
-          ini-values: error_reporting=E_ALL, display_errors=On
+          ini-values: error_reporting=E_ALL, display_errors=On, display_startup_errors=On
           coverage: none
 
       - name: Conditionally update PHPCompatibility to develop version