[3.0] Use PCOV to avoid segfault with Xdebug 3.4.2 on PHP 8.0+

Builds on #223, #222, #218, and bases on https://github
.com/reactphp/async/pull/92 .

Author: WyriHaximus

.github/workflows/ci.yml

@@ -25,7 +25,7 @@ jobs:
       - uses: shivammathur/setup-php@v2
         with:
           php-version: ${{ matrix.php }}
-          coverage: xdebug
+          coverage: ${{ matrix.php < 8.0 && 'xdebug' || 'pcov' }}
           ini-file: development
       - run: composer install
       - run: vendor/bin/phpunit --coverage-text