[BUGFIX] Apply workspace context in FileRepository::findByRelation

FileRepository::findByRelation does not apply any workspace context
when invoked in backend context.

Resolves: #59334
Releases: main, 11.5
Change-Id: I98f3f0640773c7d87879adb2b75cdb92a410bbc3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74503


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>