[BUGFIX] max_execution_time check fails

Fix for wrong error message in system environment check. Checking
max_execution_time fails in cli mode.

Resolves: #48095
Releases: 6.1, 6.2
Change-Id: I5cfe59dfab680d49a2b1b023a09e2d8d458345f4
Reviewed-on: https://review.typo3.org/20709
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind