[BUGFIX] EXT:form - fix templateRootPaths support for email finishers

Fixes "templateRootPaths" support for email finishers.
"templatePathAndFilename" is still functional to avoid breaking changes.

Resolves: #80974
Releases: master, 8.7
Change-Id: Ic8055a925c0608a9166f98b52c34435ac2a56082
Reviewed-on: https://review.typo3.org/53365


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Frans Saris <franssaris@gmail.com>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Tobi Kretschmann <tobi@tobishome.de>
Reviewed-by: Sascha Rademacher <sascha.rademacher+typo3@gmail.com>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>