bug #446 [Maintenance] Add flex support to global composer (lchrusciel)

This PR was merged into the 1.9 branch.

Discussion
----------

| Q               | A
| --------------- | -----
| Bug fix?        | yes
| New feature?    | no
| BC breaks?      | no
| Deprecations?   | no
| Related tickets | Continuation of #460, replacement of #463
| License         | MIT

Commits
-------
ea983d4 [Maintenance] Add flex support to global composer

.github/workflows/build.yml

@@ -56,6 +56,7 @@ jobs:
                 name: Restrict Symfony version
                 if: matrix.symfony != ''
                 run: |
+                    composer global config --no-plugins allow-plugins.symfony/flex true
                     composer global require --no-progress --no-scripts --no-plugins "symfony/flex:^1.10"
                     composer config extra.symfony.require "${{ matrix.symfony }}"
                     (cd src/Component && composer config extra.symfony.require "${{ matrix.symfony }}")