[GitlabCI] run-tests.sh does not pass on SYMFONY_DEPRECATIONS_HELPER environment variable value to spawned processes
Problem/Motivation
In contrib, using GitlabCI pipelines with the templates, the content of the SYMFONY_DEPRECATIONS_HELPER environment variable is taken into account when _PHPUNIT_CONCURRENT is set to 0 (hence PHPUnit CLI is executed), but not when it is set to 1 (hence run-tests.sh is executed.
See https://drupal.slack.com/archives/CGKLP028K/p1699433881643249
Proposed resolution
Fix run-tests.sh so that it passes through the value of SYMFONY_DEPRECATIONS_HELPER to spawned processes.