[TASK] Improve type annotation for GU::setIndpEnv

This helps static code analysis in extensions.

Resolves: #97893
Releases: main, 11.5
Change-Id: Ie2eee8244438cc8d4624608ce261f5418540c672
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75097


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>