[TASK] Use Environment API instead of PATH_site in EXT:backend

Several places of the PATH_site constant in EXT:backend are replaced
by the new Environment API.

Resolves: #85272
Releases: master
Change-Id: I992b07a5bde377f9a65798eaf9ec797433eb9ce4
Reviewed-on: https://review.typo3.org/57229


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Jan Helke <typo3@helke.de>
Tested-by: Jan Helke <typo3@helke.de>