[BUGFIX] Skip main extension repository check in Composer mode

Extensions are not installed and updated from the main extension
repository in Composer mode, thus skip this check completely.

Resolves: #79277
Releases: master, 7.6
Change-Id: I6cb209b5ebae04b7e29b55a52ac61ce0f4b261c2
Reviewed-on: https://review.typo3.org/51267


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>