Global `symfony/flex` is not needed

[asispts]

symfony/flex is already added to composer.json https://github.com/symfony/demo/blob/0ff22b52b6ec3e0217d33f0a12a66020ec277ed8/composer.json#L29

So, I think this action is not required. https://github.com/symfony/demo/blob/0ff22b52b6ec3e0217d33f0a12a66020ec277ed8/.github/workflows/tests.yaml#L58-L59

The action will also produce an unnecessary error

Btw, why run composer update instead of composer install? https://github.com/symfony/demo/blob/0ff22b52b6ec3e0217d33f0a12a66020ec277ed8/.github/workflows/tests.yaml#L61