Ignore existing env vars when running dump-env

[nicolas-grekas]

Fix #475
Fix #476
Fix symfony/symfony#30418

[bendavies]

didn't get a notification for this on my linked issue. thanks - i'll take a look.

[DavG]

This introduces bug #500

foreach ($_SERVER as $k => $v) {
    if (\is_string($v) && false !== getenv($k)) {
        unset($_SERVER[$k]);
        putenv($k);
    }
}

[bendavies]

@nicolas-grekas I thought I checked but it doesn't not seem that this fixed the issue described at #476.

https://github.com/bendavies/symfony-flex-env-issue is still a problem.

[bendavies]

looking at the fix in this PR, I cant see the issue was resolved actually.
the problem was not with dump-env, but with the loading of var in bootstrap.php