ci: Move back to manual composer setup, and fix command

Author: exaby73

.github/workflows/tests.yml

@@ -28,10 +28,11 @@ jobs:
         with:
           php-version: ${{ matrix.php }}
 
-      - name: Install dependencies
-        uses: php-actions/composer@v6
-        with:
-          command: export SYMFONY_REQUIRE=${{ matrix.sf_version }} composer update
+      - name: Install symfony/flex
+        run: composer global require --no-progress --no-scripts --no-plugins symfony/flex
+
+      - name: Composer update on php ${{ matrix.php }} and symfony ${{ matrix.sf_version }}
+        run: export SYMFONY_REQUIRE=${{ matrix.sf_version }} composer update
 
       - uses: php-actions/phpunit@v3
         with: