[BUGFIX] Ensure Install Tool is not using a deprecated property

The install tool still uses a variable which is deprecated.

Instead, the install tool already defines absPrefix as PATH_site which
is the way to go here.

Resolves: #80690
Releases: master, 8.7
Change-Id: Ib87e0a91e6e7b2151e019881dd137a0b011ae488
Reviewed-on: https://review.typo3.org/52527


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>