merge 4.4 to 5.4 (allow composer to run the symfony/flex plugin)

[alexislefebvre]

follow-up of #12

It should fix CI issues like in https://github.com/symfony/symfony-docs/runs/7303736198?check_suite_focus=true

The logs will be erased in the future, here are the logs for further reference:

git clone -b 5.4 --depth 5 --single-branch https://github.com/symfony-tools/symfony-application.git _sf_app
  cd _sf_app
  composer update
[…]

Error: symfony/flex contains a Composer plugin which is blocked by your allow-plugins config. You may add it to the list if you consider it safe.
You can run "composer config --no-plugins allow-plugins.symfony/flex [true|false]" to enable it (true) or disable it explicitly and suppress this exception (false)