[BUGFIX] Ensure absolute paths for &eID=dumpFile

When using realurl (or other extensions modifying the
path of the frontend) then index.php may be linked not
to PATH_site and needs to be taken into account.

Resolves: #81486
Releases: master, 8.7
Change-Id: Ie58fd7f020d26d936c4ce313edb359c3ed9831ef
Reviewed-on: https://review.typo3.org/53140


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>