[BUGFIX] Nested FLUIDTEMPLATE doesn't work anymore

It's impossible since version 6.0 to render a
FLUIDTEMPLATE content object inside a
FLUIDTEMPLATE, because the internal fluid view
instance of the parent FLUIDTEMPLATE will be
overridden by the children FLUIDTEMPLATE.

Fixes: #48021
Releases: 6.2, 6.1, 6.0
Change-Id: I776a3b0a388bb47d2995041679e39ec7442459c2
Reviewed-on: https://review.typo3.org/20585
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind