[TASK] Mark SystemEnvironmentBuilder as @internal

This class has a human-readable comment marking it internal.
Now it also has the corresponding PHPDoc annotation so IDEs
can recognize the class as being internal, too, so that developers
using the class can see a warning in their IDEs.

Resolves: #100690
Releases: main
Change-Id: Id9b7e3b52829a36af796801c3723541f83b0213c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78774


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christoph Lehmann <christoph.lehmann@networkteam.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christoph Lehmann <christoph.lehmann@networkteam.com>