[FEATURE] Allow to disable the "Core Updater" through an env-variable

By setting the environment variable "TYPO3_DISABLE_CORE_UPDATER=1" the
Core Updater functionality is disabled in the Install Tool. This can
be useful for providers which have it's own means of upgrading the
Core.

Resolves: #52639
Releases: 6.2
Change-Id: I88272ea613d95512ad780cefa5e261cf2dc6a60c
Reviewed-on: https://review.typo3.org/24629
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring