[BUGFIX] Missing typo3temp/var/transient breaks thumbnail generation

The generation of preview images fails because the directory
typo3temp/var/transient does not exist and is not automatically created.

Change-Id: I905ebc5e4e886276eb907221c09fbe582119a148
Resolves: #74200
Releases: master
Reviewed-on: https://review.typo3.org/47081


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>