Use Composer v2.8 in CI

v2.9 runs audit checks which fail for the laminas/laminas-diactoros for
PHP 7.4. However the command line config command for Composer 2.9 hasn't
yet been released, so we can't easily configure during CI.

Author: akrabat

.github/workflows/tests.yml

@@ -35,6 +35,7 @@ jobs:
         with:
           php-version: ${{ matrix.php }}
           coverage: xdebug
+          tools: composer:2.8
 
       - name: Install dependencies with Composer
         run: composer update --prefer-dist --no-progress --no-interaction --ansi ${{ matrix.composer-options }}