Fix flex SYMFONY_REQUIRE configuration in CI

[GromNaN]

Flex uses the SYMFONY_REQUIRE variable for version constraint https://github.com/symfony/flex/blob/f356aa35f3cf3d2f46c31d344c1098eb2d260426/src/Flex.php#L129

[GromNaN]

I made the wrong assumption about the purpose of SYMFONY_VERSION; it's used for testing.

maker-bundle/src/Test/MakerTestEnvironment.php

Lines 405 to 408 in c86da84

	public function getTargetSkeletonVersion(): ?string
	{
	return $_SERVER['SYMFONY_VERSION'] ?? '';
	}