[BUGFIX] Repair f:image VH for Backend

ImageMagick command was executed from directory
typo3/, but output file is relative and starts
with typo3temp.
This patch adds absolute path to output file
when in BE mode.

Resolves: #54886
Releases: 6.2
Change-Id: I6216a9bb74b9619c090c1ef70322cb6788a73c92
Reviewed-on: https://review.typo3.org/26729
Reviewed-by: Oliver Klee
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers
Reviewed-by: Marcin S?gol
Tested-by: Marcin S?gol
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind