Symfony version(s) affected: 4.1.5

Description

When I define env variables in the phpunit.xml.dist:

    <php>
        <env name="MY_ENV" value="Tests"/>
    </php>

the env are not available when giving a directory for run parallel testsuites:

How to reproduce

bin/simple-phpunit path

echo getenv('MY_ENV'); // false

The following works:

bin/simple-phpunit -c path/to/phpunit.xml.dist

echo getenv('MY_ENV'); // returns 'Tests'

EDIT Repository to reproduce the error: https://github.com/alexander-schranz/symfony-phpunit-test

Possible Solution

Currently not sure as I did not foundout how the parallel tests are done? see #28995