[BUGFIX] Mitigate thread unsafe usage of getenv

Drop usage of getenv('TYPO3_PATH_COMPOSER_ROOT') and use
Composer provided runtime API to determine the path of the
root package.

This mitigates thread unsafety at least for packages, which
is more prevalent than other getenv usages, that exist in TYPO3
already. These should be tackled in subsequent changes.

Resolves: #95171
Related: #94996
Releases: master
Change-Id: I7e45aaf72d24ac3575488b708220ba754cc131e6
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71075


Tested-by: Benjamin Franzke <bfr@qbus.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>