[BUGFIX] Failing task "Fileadmin garbage collection"

Fix the identifier string, which is used to retrive the file object,
a relative path instead of an absolute path is expected.

Releases: 7.6
Fixes: #82672
Change-Id: I8786330f07996f1b341bb38fafa50709d364d25e
Reviewed-on: https://review.typo3.org/54839


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Manuel Selbach <manuel_selbach@yahoo.de>
Tested-by: Manuel Selbach <manuel_selbach@yahoo.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>