[BUGFIX] Add formDataProvider DatabasePageRootline to flexFormSegment

The variable ###SITEROOT### can only be correctly substituted if the
DatabasePageRootline provider is used. There it is added to the
flexFormSegment as well.

Resolves: #81022
Releases: master, 9.5

Change-Id: I49ecc8cb3c98b7436a013c090618960368f2b37a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63594


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Christian Eßl <indy.essl@gmail.com>
Tested-by: Susanne Moog <look@susi.dev>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Christian Eßl <indy.essl@gmail.com>
Reviewed-by: Susanne Moog <look@susi.dev>