Use Flex tool from setup-php action to lock Symfony version

[mbabker]

The shivammathur/setup-php action provides Flex as a global tool when setting up the environment. So instead of installing it through an extra step, we can use the resources the action already provides.

Next, setting the SYMFONY_REQUIRE env var is the same as writing the extra.symfony.require key in composer.json.

All told, we can avoid a couple of extra Composer commands by using resources that are already available. It's a minor optimization, but it's one nonetheless.

[loic425]

@lchrusciel it works => https://github.com/Sylius/PluginSkeleton/runs/6894864414?check_suite_focus=true#step:15:15

[Zales0123]

The base of this pull-request was changed, you need fetch and reset your local branch
if you want to add new commits to this pull request. Reset before you pull, else commits
may become messed-up.

Unless you added new commits (to this branch) locally that you did not push yet,
execute git fetch origin && git reset "patch-1" to update your local branch.

Feel free to ask for assistance when you get stuck 👍

[GSadee]

Thank you, Michael! 🎉