Add symfony/flex to the list of Composer plugins allowed to run

[ste93cry]

As per title, when we added the allow-plugins option to composer.json we forgot the symfony/flex plugin, which is required to install the same version for all Symfony components in the CI. This PR fixes the issue

[ste93cry]

Apparently, adding the plugin to allow-plugins in our composer.json is not enough and we must add it to the global one instead because the tool is installed using shivammathur/setup-php. I've opened a bug report in shivammathur/setup-php#611 because I think that the GitHub action should do it by itself

[ste93cry]

Closing because no more needed. A new release of the GitHub Action fixed the problem 🥳