[BUGFIX] clearCache.system does not show up

New TSconfig option clearCache.system does not work as expected
because class \TYPO3\CMS\Backend\Toolbar\ClearCacheToolbarItem
wrongly checks access to that option. Thus if only clearCache.system
is set the clear cache menu does not show up at all.

Resolves: #57064
Releases: 6.2
Change-Id: I51c9605c830a9099f21d254ae86d8d6cc430b02a
Reviewed-on: https://review.typo3.org/28518
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Wouter Wolters
Reviewed-by: Kay Strobach
Tested-by: Kay Strobach
Reviewed-by: Francois Suter
Tested-by: Francois Suter